Cassiterite

Cassiterite is tin oxide - the primary and almost sole ore of tin, one of the first metals smelted by humans. Tin's role in creating bronze (an alloy of copper and tin) makes cassiterite one of the most historically important minerals in human civilization. The Bronze Age (roughly 3300-1200 BCE) was only possible because ancient peoples figured out how to smelt cassiterite.

Cornwall, England, was one of the ancient world's major tin sources. Phoenician traders sailed from the Mediterranean to Cornwall for tin, creating one of the longest trade routes of the ancient world. The 'Cassiterides' (Tin Islands) were mentioned by Greek historians as a mysterious source of tin at the edge of the known world - almost certainly referring to Cornwall or the Isles of Scilly.

Cassiterite forms in high-temperature hydrothermal veins and pegmatites associated with granitic magma. Its extreme density (6.99) causes it to concentrate in alluvial placer deposits, which were the easiest ancient source to exploit.

Cassiterite is identified by its dark brown to black color, extreme density (6.99 - very heavy), adamantine luster (almost diamond-like shine), and tetragonal crystal habit. The combination of dark color with brilliant adamantine luster is striking.

Distinguish from magnetite (magnetic, slightly less dense), hematite (red-brown streak vs white), and black tourmaline (different luster, striated). Cassiterite's adamantine luster on dark surfaces and extreme heft are its best diagnostic features.