Crystal structure "white tin" respectively. Two more allotropes, γ and σ, exist at temperatures above 161 °C and pressures above several GPa. White tin is metallic, and is the stable crystalline form at or above room temperature. Below 13.2 °C, tin exists in the gray form, which has a diamond cubic crystal structure, similar to diamond, silicon or germanium. Gray tin has no metallic properties at all, is a dull gray powdery material, and has few uses, other than a few specialized semiconductor applications. Although the α–β transformation temperature of tin is nominally 13.2 °C, impurities (e.g. Al, Zn, etc.) lower the
Dartmoor tin-mining documentary evidence also relates: that there was a boom in tin production in the 13th century. The earliest written record of tin streaming comes from the 12th century. By the 15th century, the sources of stream tin were becoming scarce and since the demand for tin was as strong as ever, the direct working of lodes was considered viable despite the greater effort needed in both extraction and refining. "Beamworks" was the name given by the tinners to workings where the lode was followed by digging down from the surface, and the massive gullies that resulted remain prominent features of