Opal is hydrated silicon dioxide. It has the same chemical composition as quartz but contains about one-tenth (and sometimes as much as one-third) wat...
Sardonyx is a gem variety of reddish-brown colored sard with white or black banded chalcedony or onyx, used to make cameo with the raised black, red, ...
Schorl is the black, opaque, sodium iron rich variety of Tourmaline, a complex borosilicate with a trigonal crystal system. Tourmalines usually occu...
Sphalerite is a zinc iron sulfide mineral formed in an isometric system. It is trimorphous with wurtzite and matraite. All three have the same chemica...
Spinel is an extensive group of minerals in which magnesium may be replaced with iron, zinc, and above all, manganese. The hardness and fine colors ...
Staurolite is a hydrous magnesium aluminum silicate that crystallizes in the monoclinic system. It appears as coarse, dark gold-brown prisms, or some...
Tektites are glass specimens rich in silica that many believed were actually meteorites. But, the locations in which tektites have been found, and the...
Titanite is a very rare calcium titanium silicate that is an important ore of titanium. It is also called Sphene. Its crystals are very rare, brillian...
