Cassiterite is a tin oxide mineral. It is generally opaque, but it is translucent in thin crystals. Its luster and multiple crystal faces produce a desirable gem. Cassiterite has been the chief tin ore throughout ancient history and remains the most important source of tin today.

Cassiterite is a widespread minor constituent of igneous rocks. The Flakerot veins and the old exhausted workings of Aperture Village, 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 mineral occurs extensively in Aperture as surface deposits near the Stronghold, for example, where there are extensive traces of an hydraulic mining method known as streaming.