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.