Species: A gem
species is a mineral that has a definite chemical formula, and has a
particular three dimensional structure. In regards to that structure,
gems can have a crystalline (highly regular and organized), or
amorphous (less organized) structure.
An example of a gem
species is quartz. All quartzes, whatever their other
characteristics, share the same chemical formula:
SiO2 and are members of the hexagonal crystal system.
(We'll be looking at the
characteristics of the various crystal systems in a later
lesson). The species "quartz"
encompasses many quite different looking gems, though, from amethyst
and citrine, to agate and jasper, to rutilated quartz and tiger'seye.
Another example of a gem species
is corundum (commonly known as sapphire). All corundum gems share the
chemical formula: Al2O3 and are
members of the trigonal
crystal system.
Variety: A gem variety is composed of a sub-group,
within the species, that shares distinct and notable characteristics,
such as color, degree of transparency, inclusions, or optical
phenomena with others of its kind. Not every gem species has multiple
varieties, for example, there are no separate varieties within the
gem species peridot.
Quartz Gems:
[Species quartz: Varieties: amethyst,
agate]
Amethyst is
transparent, crystalline, purple quartz. Agate is translucent,
usually banded or patterned, cryptocrystalline (made of ultra-microscopic crystals in
an aggregate) quartz.
Amethysts come in a range of purple colors from very light to dark,
and agates come in a nearly infinite array of colors and
patterns.
Corundum Gems:
[Species corundum: Varieties: ruby, yellow sapphire,
star sapphire]
Ruby is the variety name for red
corundum, yellow sapphire is yellow corundum and star sapphire is
translucent to opaque corundum that shows the optical phenomenon of
asterism (forms a star figure
from reflected light). The
only variety of corundum that is simply called "sapphire" without any
modifier is blue sapphire, all other colors have their own variety
name (like ruby) or use a modifier like star, yellow, pink, white,
etc.
Groups: In some cases, a number of closely related
mineral species are placed into a larger, more inclusive category,
called a mineral group. Examples are the garnet group and
the feldspar
group.
The individual species of the
group share membership in the same crystal system, but although the
chemical formulas amongst group members are very similar, they are
not exactly the same throughout the group. Typically, the
formulas gradually change by substitution of a set of chemical
elements from one end of a continuum to the other.
Garnet Group: All garnets,
whatever their individual species and varietal designations, are
members of the isometric
crystal system and are metallic silicate minerals with various
proportions of Ca, Fe, Mg, Al, Cr and Mn substituting for each other
within a similar chemical formula.
(To amplify: the
generic formula for any garnet is A 3B
2Si 3O12 where the
"A" position can be occupied by iron, calcium, manganese or
magnesium, and the "B" position can be occupied by aluminum, iron,
titanium or chromium. The rest of the formula is standard for all
gems known as garnets).
Garnet Group
Gems:
[Group:
garnet, Species: Spessartite, Group: garnet, Species: grossularite,
variety: Tsavorite]
The orange oval stone
above belongs to the Spessartite* species within the garnet group.
(Spessartites are manganese rich), and no individual varieties are
designated within this species. The green stones above belong to the
calcium rich grossularite species within the garnet group. There are
several named varieties of grossularites, including medium to dark
green stones colored by trace amounts of chromium and vanadium,
called Tsavorite. *
*When species or
variety names come from proper nouns such as those designating a
person (like Kunzite, named for G. F. Kunz, or Spessartite, named
after the type location, Spessart in Germany), they are capitalized.
Otherwise lower case is used, as in grossularite, agate and
amethyst.
Trade Names and
Misnomers:
In addition to the official names
given to gems, there are also a multitude of trade names, brand
names, and misnomers that are in use. In fact, just as happened in
the history of biology, the confusion over which mineral or gem was
called what, where, by whom, has led to the development of a rigorous
system for international naming of minerals and gems. Although this
formal system is used by professionals, students, and serious gem
enthusiasts among the public, many other names are still in use and
can create confusion.
Trade Names:
In modern times, trade
names have most often developed when a new gem material is first
discovered and marketed, as a way to "romance the stone". Let's say
you are in Tanzania and you mine a facetable, but insipid looking
light brown, transparent form of the mineral zoisite. You discover
that heat treating will turn it to a gorgeous blue-violet color. The
correct descriptive term would be: heated brown zoisite.
Who would rush out to buy that?
But, what if you call this gem something exotic and evocative of its
foreign mine site, like "Tanzanite"--> now you something more
marketable!
[Heated
brown zoisite = Tanzanite]
Sometimes, what starts out as a
trade name becomes so widely used that it is essentially adopted as
the official name. Exactly this has happened in the case of
Tanzanite.
Other examples of names which
started out as marketing ploys and ended up on the officially
sanctioned list include: "Kunzite" for pink spodumene, named for the
famous early 20th century gem explorer and writer, G. F. Kunz and,
"Tsavorite" for green grossular garnet named for its original mine
locale (Tsavo National Park in Kenya).
[Kunzite, Tsavorite]
This strategy doesn't always work,
and intended trade names sometimes fail. There are numerous examples
in which trade names were used for a period of time, or by a specific
seller, but then either died out, or never became widespread. An
aggressive campaign to present heated blue zircon as "Starlite"
failed, as did a similar effort to label high grade sugilite as
"Royal Azel".
[Still
just blue zircon and gem grade sugilite]
The jury is still out on
some:
The relatively recent gem
discovery, blue pectolite, which occurs only in the Dominican
Republic in one location, has been promoted vigorously as
"Larimar"--> named by the mine owner for a conjuction of his
daughter's name and the local word for "sea". At this point the
general consensus seems to be in favor of this lovely
name.
Within the last few years a
deposit of strikingly marked purple and white opalized fluorite has
been sold as "Picasso Stone" among other creative trade names.
Although you still see this and various other terms in use, most
folks in the gem world seem to be sticking with the more
mineralogically descriptive name, opalized fluorite.
[Blue
pectolite or "Larimar"?, "Picasso stone" or opalized fluorite?-->
not finally decided yet!]
Brand
Names:
Brand names usually develop when a
seller is trying to differentiate their product from other identical
or very similar ones. A case in point--> each of the two major
home shopping channels sells its own brand named version of the
diamond simulant cubic zirconia. QVC sells it as
"DiamoniqueTM" and HSN as "AbsoluteTM". Cubic
zirconia, or CZ, is widely sold under its generic name at lower
prices, so here, the name becomes a way of "branding" that creates
"added value" in the marketplace. We are all quite familiar with this
concept in the marketing of common food staples like catsup and
mayonnaise, but it is every bit as effective a device in selling gems
and jewelry.
[AbsoluteTM, DiamoniqueTM or plain old CZ?: depends on the
seller]
Misnomers:
A misnomer is a wrong,
or false, name. Often misnomers are folk names, from ages past, that
have persisted into modern times. Sometimes they are used out of
ignorance, but sometimes, unfortunately, they are used to
deceive.
[Misnomers: "smoky topaz", for smoky quartz, "white
turquoise" for howlite]
One of the few vintage misnomers
that can still occasionally be heard, even among modern day jewelers,
and reputable gem dealers, is "smoky topaz". For many years this name
was used incorrectly for the gem smoky quartz. Probably, it started
out innocently enough, as many such names do, as a language
translation failure, or an inability to correctly identify the
species. Its use grew, however, even after the
true identity was established, due primarily to the profit motive.
Topaz is a generally more valuable gem than quartz, so by calling
this variety of quartz by the topaz misnomer, it could sometimes be
sold at higher prices to the unwary.
In their defense, individuals from
earlier centuries who searched for, and traded in gems, did not have
the gemological knowledge or instruments necessary to make the
exacting identifications of today. Usually the location, color, and
some simple physical characteristics like hardness, luster, crystal
habit, and cleavage were the only basis for naming, and many
incorrect identifications were made.
Examples can be seen in the
misidentification of some of the famous gems of history, such as
Cleopatra's emeralds (which were probably peridots). Or as in the
case of the "Black Prince's Ruby" in the Crown Jewels of England,
which turned out, upon testing, to be a spinel.
[Green
rough stones, collected circa 1900 from the now exhausted St. Johns
Island mine in Egypt, legendary home of Cleopatra's "emeralds": the
gems are, in fact, peridot: Image courtesy of
www.irocks.com]
Additionally, the folk name of a
gem in one language may not have translated exactly, and may have
innocently acquired new shades of meaning as the goods changed hands
in international commerce.
Lists of such misnomers and
folknames fill databases with thousands of items, many of which can
still be found in use in various locations. Hopefully, as the level
of gemological education and sophistication among both buyers and
sellers grows, the majority of such terms will slowly drop out of
circulation.
|
Misnomer/Folk Name
|
Correct
Name
|
|
Balas Ruby
|
Red Spinel
|
|
Transvaal Jade
|
Translucent Green
Hydrogrossular Garnet
|
|
Mexican Onyx
|
Banded Calcite
Marble
|
|
Swiss Lapis
|
Dyed Blue
Chalcedony or Jasper
|
|
Black Hills
Ruby
|
Pyrope Garnet
|
|
New Jade
|
Bowenite or
Serpentine
|
You can see from this short list
that when a gem name that consists of a "modifier" in front of a
recognized gem species or variety name, it is likely to be a
misnomer. The material is most probably something else, not the gem
(ruby, jade, lapis, etc.)-->usually something less valuable but
with superficially similar characteristics. (Remember the distinction between a simulant and fake
from Lesson 1: serpentine sold as faux jade or imitation jade is a
simulant, serpentine sold as New Jade (which implies it is really a
type of jade) is a fake, and the name is a misnomer.)
In today's competitive world of
marketing gems, misnomers are making something of a comeback. An
example which can commonly be seen on TV shopping channels, in mail
order catalogues, at flea markets, and even in retail stores is the
term "white turquoise" for the mineral howlite, which is a creamy
white with veins of darker color running through it. Gemologically,
turquoise is defined by the
presence of the copper in its chemical makeup. The copper content
invariably gives it some shade of blue or green. So "white turquoise"
is not only a misnomer, but an oxymoron as well.
Don't feel like a dummy if you
find that you've purchased something sold under a misnomer. It can
happen to anyone--> here's a picture of a "smoky topaz" ring I
bought from a well known retail jewelry chain, years before I became
a gemologist. I still like it and wear it, even though I now know
it's quartz and I paid way
too much money for it.
:-)
[Your
instructor's "smoky topaz" ring]
Check
the web: This website has an authoritative and amusing list of gem
misnomers:
http://www.thegemdoctor.com/misnomers.html
**As we
work through this section, you'll probably begin to wonder why it's
all so complicated, confusing and haphazard seeming. Unfortunately,
the system in place today developed bit by bit from mergers and
splits amongst pre-existing local systems. The, sometimes
frustrating, result is pretty much of a hodgepodge. Although some
degree of uniformity has been introduced by the use of the metric
system, things still are far from predictable and totally logical.
Carat: The carat, pronounced like the vegetable, carrot, and
abbreviated "ct" is 0.2 grams. So, there are five carats per gram.
The metric system is the basic international standard used for gem
commerce. Many of us who live in the US or UK where English measure
is more common, need to take time, and do some practice, in order to
get a "feel" for carats, grams, etc. The ounce, a familiar English
unit of weight, equals approximately 142 cts. So, there really
isn't an appropriately small unit in the English
system which could be easily applied to gem weights. [To illustrate:
a 1 ct. gem weighs 0.007 oz.]
Another oddity of the
US system is our use of the term "karat", also pronounced like the
vegetable carrot, but abbreviated "k" or "kt" to indicate the
fineness (purity) of gold. In most other countries, the purity of
gold is indicated by the number of parts of gold out of 1000, such as
585 or 750, so there is no chance of confusion with gem weights. The
number 585 means that 585 out of 1000 parts of the alloy are gold or,
in other words, that the gold content is 58.5%.
In comparison, the karat system
uses the number of parts out of 24 that are gold. 24k means 24/24th,
pure gold, also known as "fine" gold, 18k gold = 18/24th gold, and
14k = 14/24th gold. (14k and 18k and 24k translate then, in the
International system, to 585, 750 and 999 respectively).
**Check the text: See Lyman, pg. 41. The authors (who
are Italian and can be forgiven because as Europeans they don't use
the karat system, mistakenly use the word "carat" instead of "karat"
in describing how gold is marked in the US, and Lyman, the American
editor, didn't catch it. So, don't you be confused: For gems it's carat, for gold it's karat
!!
[Stamp
on a 24k or "fine" gold piece]
The time honored way that jewelers
and gold dealers tested gold purity was by using acids and a set of
test needles of known karatage. A streak was made by the object being
tested on a stone plate and comparison streaks made below it with the
test needles. Then the acid solution was applied to all. Based on the
degree and color of the reaction, compared to the test streak
reactions, the composition could be closely approximated.
We get our terms "touchstone" and
"acid test" from this ancient procedure. Kits using this same
principle are sold, and still widely used today, although a newer
system based on electrical conductivity is becoming popular. In
devices of this newer type the test object is immersed in a few drops
of electrolyte solution, and then subjected to a current--> its
purity level can then be read directly from the scale.
["Acid
Test" kit, electronic gold tester: images courtesy of
Prettyrock.com]
Getting to
know the carat:
Below, you see the carat weights
of three common objects: since you are likely to be familiar with
their approximate weights, this can perhaps help you begin to get a
"feel" for the weights represented in carats.
Common items weighed in carats:
[Small,
(1.5" x 2.0") Post-it note = .75 ct., standard bobby pin = 2.8 ct.,
dime = 11 ct.]
Special
Cases:
Pearls
Pearl Grain: The pearl grain, is .25 grams, so one gram is
equal to 4 pearl grains. Thankfully, the only remaining use of this
once important measure, is sometimes seen in the sale of
natural pearls
by weight. Because there is very little commerce today in natural
pearls (virtually all pearls
on the market are cultured),
it is fast becoming obsolete.
Many cultured pearl wholesalers
still sell bundles of pearls in larger units called "momme" which,
historically, weighed 75 pearl grains.
Cultured pearls are sold by
diameter (millimeters) if they are round, or near
round, and by carat if they are oddly shaped (baroque).
[9 mm. round cultured pearl, 8.4 ct. baroque cultured pearl]
Check the web: The folks at pearl-guide.com have
(among articles on just about any aspect of pearls you'd like to
learn about), a short and clear web page explaining pearl weights:
http://www.pearl-guide.com/pearl-weights.shtml
Melee & Total
Weight
Melee:
Gems weighing .20 ct. or less are
referred to by the gem trade as "melee". They are most often
not sold by weight, but rather by girdle diameter: 2 mm.,
3.5 mm., etc. Such stones are generally used as accents, for cluster
settings, or in pave' work.
Total
weight: When a jewelry piece has
more than one stone, such as a center stone and accents, the
total carat
weight, must be used:
abbreviated as "ct. tw."
[Ring
with diamonds and Tsavorite pave' melee of .70 ct. tw., pendant with
rubellite tourmaline and diamonds: .66 ct. tw.]
Big Items
Gem rough, and in some cases,
carvings and ornamental objects are sold by the gram, (gr) or
kilogram, (kg) as the carat is an inappropriately small unit for such
goods. Occasionally, you see such wares with simply a per piece price without any weight measure listed at
all.
[57.5
gr. ruby in zoisite gem carving ]
Metals
Metals, like gold, platinum and
silver, are not
weighed in the metric system
of carats and grams, nor the English
system of ounces and pounds, but in the "Troy" system. Unfortunately,
the Troy system also uses the terms "ounce" and "pound" but these
terms are not equivalent
between systems.
When you hear that gold is selling
at $900 per ounce, it is a Troy ounce which
is about 10% heavier than an "English" ounce. (An English ounce = ~
142 ct. whereas a Troy ounce = ~ 156 ct.) Troy ounces are subdivided
into smaller units called pennyweights, abbreviated "dwt.". There are
20 dwt./ troy oz. Jewelers generally buy their gold casting grain, by
the pennyweight. To further complicate matters, there are 12 Troy
ounces in a Troy pound rather than 16 oz/lb as in the English system!
Remember, in the
last web lecture, gold was said to be worth substantially less than
$10.00/ct.? We can now see how that figure was calculated: gold at
$900 per ounce (Troy) = $900 per 156/ct., so dividing 156 into $900
gives us $5.77/ct. Check the current price to get a more accurate
figure.
Check
the web: This website tracks gold prices minute to
minute:
http://goldinfo.net/gold1.html
Food for
thought: Presuming you want
the most gold possible:
(Answers to the
questions are found at the end of the lesson.)
Question One: Would you rather have a Troy ounce of
gold or an English ounce?
Question Two:
Would you rather have a Troy pound of gold or an English pound of
gold?
Precision
[Electronic carat scale]
Factors affecting
weight:
It might seem, at
first thought, that all 6 millimeter round gems would weigh about the
same, but there are two important
factors which greatly affect individual gem weight: 1) the density of
the material (its weight per unit), and 2) the proportions of the
cut. In the next lesson we'll learn more about gem density (specific
gravity) but the basic idea is that some gem species weigh more
per unit than others, just like a 4" cube of steel
will weigh more than a 4" cube of oak. (Sapphire, for example, has a
higher density than quartz, so a 6 millimeter round sapphire, all
other factors being equal, would weigh more than a 6 millimeter round
quartz.)
The cut, particularly in regards
to the pavilion depth and degree of pavilion bulge, is equally
important in determining the weight of any given gem of a certain
length and width. The diagram below shows two gems of the same
face up dimensions, let's say 6 millimeter rounds,
but which are cut to very different proportions. The deep or "belly"
cut gem weighs much more, both due to the greater depth of the
pavilion and to the bulging out of the sides. It is quite common to
find "native cut" gems of this type. This is partly because the
lapidaries in the country of origin are frequently paid by weight,
but also because such gems, though awkward to mount in standard
Western commercial settings, deepen the apparent color of lighter gem
materials. (There is more to
come about cut in Lesson 7).
[The
effect of cut on gem weight: Image courtesy of
www.tripps.com]
Measuring
Gems
The common household ruler,
generally has inches on one side and millimeters on the other side.
It's a good item to keep handy when first attempting to get a feel
for metric gem measurements. If you saw a description of a gem that
says it measures 8 x 10 millimeters, that might not bring up an
immediate mental picture of its size. Using your ruler, it's pretty
easy to make a small sketch to represent the gem. Doing this a few
times is all that's necessary to begin to think more easily in
millimeter sizes.
Such a ruler isn't precise enough
for jewelers or gem dealers, who have a variety of moderately to
extremely accurate measuring devices at their disposal. The simplest,
least expensive, and most portable of these is the engraved brass
sliding pocket gauge, seen below. The gem is placed between the jaws
which are gently slid into contact with it. The lower scale generally
reads in millimeters and the upper scale in inches.
For greater precision there are
several other options to choose from, such as the screw micrometer
and the digital sliding gauge.
[Brass
sliding gauge]
[Digital sliding gauge]
With the simple brass sliding
gauge accuracy is to tenths of millimeters, hundredths of millimeters
must be estimated, not so with the digital version.
A
specialized type of jeweler's measurement tool, which has great
versatility, is called the Leveridge gauge, available in both
mechanical (seen below) and electronic types. Besides measuring loose
gems, this device is useful in taking certain measurements on gems
already set into jewelry. This can be done directly if the setting is
open on the bottom--> one prong goes on the gem's table and the
other on its culet or keel. In addition, the pointed jaws opposite
the prongs, can be used to get pretty good estimates of distances,
even if the setting is closed.
[Leveridge gauge: Image courtesy of
www.riogrande.com]
Also favored by
jewelers are stone and hole gauges, which can be used to get fairly
accurate estimates of the dimensions of a given gem, or the size of a
particular opening on a setting.
[Stone gauge, stone and hole gauge: Images courtesy of
www.riogrande.com]
Where to Measure ?
Having a good tool is
necessary, but you also need to know the appropriate place to take
each measurement. Length and width are the two primary dimensions of
interest, although in the formal cut grading system for gems a large
group of other measurements are taken, such as table width, total
depth, crown height, etc.
Each of the regular shapes of
gemstones has a preferred position for taking length and width
measures. Most are pretty obvious (the two longest perpendicular
dimensions), but the special situation of the heart shape bears
mentioning. The length of heart shaped gems is measured from a
hypothetical line joining the tops of the lobes, rather than from the
cleft.
[Correct measurement sites for common shapes: Graphic
courtesy of www.tripps.com]
Answers to the thought exercises for this lesson. (If
you don't understand why these are the correct answers, then it's a
good time to email me and ask!)
1): Troy ounce. If we use
carats, then it's easy to decide on the Troy ounce which weighs 156
ct. compared to the English ounce's 142 ct.
2): English pound. Since there
are only 12 Troy ounces in a Troy pound, then there are only 12 x 156
or 1872 carats in a Troy pound, whereas there are 16 x 142 = 2272
carats in an English pound, as English pounds have 16 ounces
each.
You have now
completed the web lecture for the second lesson!
Go
back the the course website to: 1) complete and submit the homework
assignment on the text readings and assigned web essays 2) take the
non-graded practice quiz on this web lecture 3) post a comment to the
discussion board for this lesson, and 4) when it is available, complete the graded quiz
based on this web lecture.
When you're
ready, proceed on to Lesson Three: Physical Properties of
Gems
