'Jet' is a well used word; we have jet engines, jet boats, jet washes and so on. Pretty well all of the common uses of 'jet' derive from the 'ejected stream of material' meaning - apart, that is, from 'jet-black'.
Jet is a hard, semi-precious stone formed by the fossilisation of wood. It may be either dark brown, with or without lighter brassy streaks or - and as far as we are concerned this is the significant characteristic - a deep glossy black. 'Jet' has been with us as a word since the formation of English and, as with many old words, it appears in early records with a myriad of spellings - 'giette', 'gete', 'jit' and so on. The earliest known reference to the stone (as 'get') is in John Lydgate's epic poem about ancient Troy, Troyyes Book, written in Middle English around 1425:
And for to telle of this eban tre,
Liche in bokys sothly as I fynde,
It cometh out of Ethiope and Ynde,
Blak as is get.
[I have found truly in books what I say about the wood of the ebony tree. It comes out of Ethiopia and India, black as jet.]
Jet was mined and fashioned into jewellery in Britain and elsewhere from at least Roman times. It takes a high polish but is difficult to work and wasn't widely used.
It was the death of Prince Albert in 1861 that changed the status of jet ornaments as items of esteem. Queen Victoria was devastated by her husband's early death and went into a prolonged period of seclusion, taking up the persona of 'The Widow of Windsor', wearing only black clothes and, tellingly for the public's perception of jet, black jewellery. She decreed that jet was the only jewellery to be worn at court for a year after Albert's death. A fashion soon arose for jet necklaces and brooches, especially for widows. Fortunately, the UK has a good supply of the stone, which is found in significant quantities at the Yorkshire coastal town of Whitby.
The fashion has now again faded in the UK at least, although it continues in other parts of Europe, and widows there no longer normally wear black clothes and jewels.
Nevertheless, 'jet-black' is still the epitome of blackness. You can have 'as black as coal', 'as black as pitch' or even 'as black as Newgate's knocker',' but for the real blackest blackness we resort to 'as black as jet'.