Sanidine—a glassy variety of orthoclase feldspar.
Satinspar—a variety of selenite or gypsum.
Selenite—a distinctly crystallized transparent form of gypsum.
Serpentine—a hydrous magnesium silicate; usually in pseudomorph forms; also fibrous, granular, cryptocrystalline, and amorphous; H. 2.5–4; luster subresinous to greasy, pearly or earthy, resinous or wax-like; feel, smooth and somewhat greasy; leek-green to blackish green and siskin green verging into brownish and other colors; apparently derived most commonly from chrysolite or olivine and also from other magnesian minerals; sometimes constitutes the bulk of rock masses.
Siderite—iron carbonate; rhombohedral; H. 3.5–4.5; luster vitreous, more or less pearly, ash-gray, yellowish or greenish, also brownish; occurs as extensive iron deposits and in crystalline rocks.
Smaragdite—a form of amphibole or hornblende (q.v.).
Spherosiderite—a globular form of siderite.
Spinel—a magnesium-aluminum oxide; crystals, octahedrons; red of various shades, passing into other colors; spinel-ruby is a variety.
Staurolite—a complex hydrous iron-magnesium-aluminum silicate; orthorhombic; disposed to cruciform shapes; occurs in schists and gneisses.
Steatite—soapstone, a variety of talc (q.v.); a hydrous magnesium silicate.