Arsenopyrite, Iron arsenic sulphide, FeAsS.

Crystal system: Monoclinic, pseudo Orthorhombic
Hardness: 5.5 to 6
Density: 6.1 gms/cm³
Colour: Silver-white to steel-grey.
Streak: Black

Arsenopyrite is the most common arsenic mineral and the principal arsenic ore. It occurs in high temperature, hydrothermal quartz veins, sometimes in association with gold. The mineral is widespread.

The specimen shown in the picture comes from Hunan Province, China and has a lovely, pale lavender, transparent 10mm cube of Fluorite on its upper surface.