Arsenopyrite
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Lustre:Metallic, Sub-Metallic Transparency:Opaque Colour:Silver-white to steel-gray, may have a slight yellow appearance Comment:Tarnished material common, some iridescent Streak:Gray-black Hardness:5½ - 6 on Mohs scale Tenacity:Brittle Cleavage:Distinct/Good Distinct on {001}; {010} in traces Fracture:Irregular/Uneven
Arsenopyrite is the most common form of arsenic in Cornwall, and is extremely common on the mine waste dumps. Part of the reason may be that it seems to degrade mush more slowly than either pyrite or chalcopyrite. Easily recognised by it's diamond shaped cross sections, it can also form long 'acicular' crystals.
As a semiconductor it is similar to pyrite, but with an overall much lower resistance. It doesn't seem to differ much across different mines or areas of Cornwall. Some negative resistance has been observed, but this needs more investigation.