“Opacity” is a term that seems to be an easy one to define. If light can not penetrate material then it is opaque. Piece of wood is opaque and most rocks are opaque. Glass, on the other hand, is not. Light shines through it, so it is transparent. Materials that are somewhere between these two extremes are said to be translucent — light penetrates them but you can not really see clearly what is behind them.
Sample of microgabbro (diabase). The width of the sample is 4 cm.
This is very common explanation but it seems to be too simplistic. Let’s take a closer look. Are rocks really opaque? What about the sample of igneous rock gabbro shown below?
We definitely can not see through this sample but what if we take a small slice of it, glue it on a glass slide and polish it until it is only 30 microns thick. This is what geologists do to examine rocks microscopically. This is how gabbro looks under the microscope:
This thin piece of rock (thin section) is placed between the observer and the light source of the microscope. Therefore, light is not reflected from the surface of the rock. These vivid colors are not real, we see them because rocks are investigated in polarized light, but this is not important now. Important point is that light really penetrates this rock. Otherwise, we would not see anything here.
So, is opacity dependent on the thickness of material? I am afraid the answer is no. Opacity is an intrinsic property of material, it can not depend on its thickness. After all, even glass will not let light shine through it if it is a kilometer in thickness.
Truly opaque materials do not let the light shine through them no matter what is the thickness. Rocks actually do contain such minerals that are opaque even if they are only 30 micrometers thick. Gold and silver are opaque minerals. Magnetite, pyrite, galenite, etc. are practically opaque as well. You won’t see through them even if they are 30 microns in thickness. So if we want to find out what is the cause of opacity, we have to focus on properties that are shared by these minerals. They actually do look similar in a certain way, they all have a metallic look. Gold and silver are pure metals and the rest of them are metallic compounds (sulfides and oxides).
Gold flakes are opaque no matter how thin they are.
So it seems that the property that is behind opacity is somehow closely associated with metals and to a somewhat lesser degree with certain metallic compounds. Metals can be defined in several ways but one of the easiest definitions is based on the type of chemical bonds: metal is a material that is held together by metallic bonds. Metallic bond is a type of covalent bond (atoms share electrons) but in metallic bond the outermost electrons are free to wonder around the material. This is what makes metals malleable, what makes them excellent conductors of electricity and heat, but it also gives them characteristic sheen known as metallic luster and it also makes them opaque.
Diamond and graphite are minerals that have a very simple composition — they are composed of pure carbon. However, despite being identical in composition, they have very different properties. Diamond is the hardest mineral. All the chemical bonds in the lattice of diamond are covalent and no electrons are allowed to wonder around. Every carbon atom is bonded to four neighboring atoms of carbon. Graphite, on the other hand, is one of the softest mineral, it contains very strong carbon sheets that are held together by weak bonds (hence the softness). Within the sheets every carbon atom is bonded (with strong covalent bonds) to three neighboring carbon atoms (sheets are two-dimensional, there is no room for four bonds). This means that every carbon atom has one additional electron with which it has nothing to do. These electrons are free to move around within the sheet of graphite. This is why graphite sheets conduct electricity and why graphite also has a somewhat metallic look. This is also the reason why graphite too is opaque.
Flakes of graphite have a metallic look and are opaque although they contain no metals.
I can not claim to understand fully how these wondering electrons make materials opaque (this is complicated physics, not my speciality) but it seems that light as an electromagnetic radiation interacts with free electrons (that are capable of carrying electric current). This interaction makes it impossible for the light to penetrate the material. It either bounces back to create a characteristic metallic luster or gets absorbed within the material.