it's part of the 'Lignite' family which is a type of coal which retains the structure of it's original 'wood' form. Technically it's a oxygenated hydrocarbon(CnH2n+2) opaque black resinous to lustrous with a hardness of 0.5-2.5
On a crystal healing level it is used to free you from fear and as a protection also a has a calming effect. Historically paired with amber in jewellery.
It's available from several different sources around the world one of the most well known being Whiteby in Yorkshire UK.