Cassiterite is a tin oxide mineral, SnO2. It is generally
opaque, but is translucent in thin crystals. Its luster and
multiple crystal faces produce a desirable gem. Cassiterite is
the chief ore of tin today.  Most sources of cassiterite today
are found in alluvial or placer deposits containing the
resistant weathered grains. The best source of primary
cassiterite is the tin mines of Bolivia, where it is found in
hydrothermal veins. Cassiterite is a widespread minor
constituent of igneous rocks. The Bolivia veins and the old
exhausted workings of Cornwall, England, are concentrated in
high temperature quartz veins and pegmatites associated with
granitic intrusives. The veins commonly contain tourmaline,
topaz, fluorite, apatite, wolframite, molybdenite, and
arsenopyrite. The current major tin production comes from placer
or alluvial deposits in Malaysia, Thailand, Indonesia, and
Russia.  Crystal twinning is common in cassiterite and most
aggregate specimens show crystal twins. The typical twin is bent
at a ne