Jet is a form of fossil and is sometimes called black amber which might be a trade name. another name is the "exorism stone".
jett is the remains of fossilized trees millions of years ago. I believe the type was a form of conifer tree. people began using jet in jewelry between 10,000-17000 B.C.E.

in the Victorian times it was used as mourning jewelry.

a general protection stone. and as it's name implies is considered to exorcise entites. wear it to help with psychic activities.

it is considered a stone of good luck for capricorns.

in ancient times it was powdered and added to wines to relieve toothaches. in modern times it is used to relieve depression.

along with amber it is used to make necklaces among witches to denote a specific rank or degree.