They are usually found in burials from the bronze age, older women usually, I was told at the museum, and more common in Scotland than elsewhere. There was a small amount of jet from Mull and those islands on West coast, but most jet came from whitby.

I'm supposing jet is sacred as an inbetween thing, neither stone, nor wood, with electrostatic properties. It would have been an easy thing to carve, at any rate. I find it easier than wood in many ways. Though more delicate.

Jet had connections with bear/serpent religious practices too. Ancient writers recorded the beleif that it protected and cured serpent bites, for instance, or could be used as a test for virginity! Little carved bear charms or talismans, or just toys (we love our teddy bears still), used to be left in graves of children (bear goddess is the foster mother of mankind).

This is just a cute bear pic, lol
http://i.treehugger.com/images/2007/10/24/playful-polar-bear-001.jpg

Jet is supposed to absorb a person's aura, the necklaces might have had something to do with that, keeping a little part of the person behind by the grave for ancestor worship, maybe. It was used as mourning jewellery much later in Victorian times the same way, but the living kept some hair from the deceased in lockets of jet while they were in mourning, so their aura was with you always, not by the grave. I've been commissioned to make replicas of those necklaces for people who have altzeimers in the family before, as they think it will help them remember who they are longer.