Cassiterite

Tin Oxide
SnO2 Cassiterite is a mineral that has ornately faceted specimens with high luster. It is generally opaque, but its luster and multiple crystal faces cause a nice sparkle. Cassiterite has been an important ore of tin for eons and is still the greatest source of tin today. Most sources of cassiterite today are not primary deposts but alluvial deposits containing weathered grains. The best source of original-formation cassiterite is at the tin mines of Bolivia, where it is found in hydrothermal veins. Although found throughout the world in many igneous rocks, cassiterite is usually only a minor constituent. The Bolivia veins and those worked and nearly exhausted in Cornwall, England, somehow concentrated the tin in a way not fully understood by geologists.

Twinning is common in cassiterite and most aggregate specimens show crystal twins. The typical twin is bent at a near-60-degree angle, forming an "Elbow Twin". Multiple twinning can continue to bend the crystal around and possibly form a cyclic twin. However, cassiterite does not form this type of twin as often as its mineral cousin, rutile.