Greensand is a sand or sandstone which owes its unusual color to a mineral glauconite. Glauconite is mixed with other sand grains in all possible proportions. Glauconite grains are usually rounded and dark green in color. Glauconitic sandstones are marine in the majority of cases. Many greensand formations seem to have formed either during the Cambrian or Cretaceous Periods.
Baltic Klint in Estonia (near Paldiski, Pakri Peninsula). It is composed of limestone (topmost layer), glauconitic sandstone (greensand), kerogene containing shale (all Ordovician) and a phosphatic Cambrian sandstone.
Closeup of a greensand (glauconite sand) from France. Width of view 20 mm.
However, greensand is not the only green sand in existence. There are several green minerals and in certain cases they may be abundant enough to give green color to the sand. Most famous example is definitely olivine. Green sand beach near the southern tip of Hawaii Island (Papakolea Beach) is world-famous but greenish beach sands containing lesser amount of olivine are not uncommon in volcanically active areas.
Is it all? No, there are a number of other green minerals. Malachite, chlorite, epidote and serpentine are all responsible for the green color of some sand samples in specific locations. But these cases are really specific and spatially very confined.
Green mineral is olivine. It is a common mineral in dark-colored igneous rocks like basalt. Papakolea, Hawaii. Width of view 20 mm.
Serpentine sand from Corse, France.
Epidote sand from a mine in Nevada, USA.