Chalcocite

A curve trace of calcocite from Penstruthal mine

• Lustre:Metallic, Sub-Metallic
• Transparency:Opaque
• Colour:Blue black, gray, black, black gray, or steel gray
• Streak:Blackish lead gray
• Hardness:2½ - 3 on Mohs scale
• Tenacity:Brittle
• Cleavage:Poor/Indistinct Indistinct on (110)
• Fracture:Conchoidal
• Comment:Somewhat sectile

Data from mindat.org

Chalcocite is also called "grey copper", and is an important ore of copper. It's fairly common, but tends to be more common as isolated crystals. Thick veins can be found, usually in a dense and 'massive' (finely crystallised) nature. It can look a bit like weathered sphalerite, but tends to be associated with green crusts of malachite and blue crysocolla.

It has a similarly low overall resistance as chalcopyrite - but it tends to have a very sudden and distinct point at which it starts conducting (around +1.3V), and when it does, it can carry large currents. Interestingly it can display properties of a Shockley diode, in that once 'switched on' it will remain in a conducting state until a much lower threshold (near to 0V) is reached.

Due to these interesting characteristics, it tends to be a favourite of mine.