Arsenic

Where it is found
Arsenic and its compounds have been known since antiquity. It occurs practically everywhere in the soil in low concentration. There are about 300 deposits worldwide. Arsenic is a trace element in antimony and copper ore. Gray or metallic arsenic is the most stable form with a density of 5,720 g/cm³. The melting point of arsenic is 817°C under a pressure of 28 atm.

What it is used for
Arsenic has a wide range of applications from pest control to fungicides in the timber industry, as rat poison, but also as a cure, and for alloying lead and copper. 

How it is traded
Arsenic is traded as a powder which must be packed in absolutely safe barrels.