Pitchstone

Pitchstone, or RETINITÉ, an acid volcanic glass, dark green, reddish brown, yellow, dark blue, or black, and occasionally showing a streaked or clouded appearance. It has a pitch-like or greasy lustre, breaks with a conchoidal or splintery fracture, and is translucent on thin edges. It is usually rich in microlites, and often contains crystalline granules and crystals of felspar, pyroxene, hornblende, biotite, and quartz. Now and again it shows perlitic and spherulitic structures. When conspicuous crystals of sanidine (see FELSPAR) are abundantly present in the rock it is termed Pitchstone Porphyry. It occurs in the form of dykes and also as lava-flows. The name pitchstone has sometimes been given to the darker varieties of menilite, a form of opal.