Jet, a gemstone often used in vintage mourning jewellery is actually fossilised wood. The black colour is caused by the high pressure during decomposition. Jet is is very light compared to most other 'gemstones' and frequently faked. How can you tell the difference?
In a beautifully worked piece of antique jewellery it's easy, look at the setting and general quality of the piece, if its of a high standard then it's 99% likely that it's real. If you are looking at beads in a necklace like the one pictured, look at the edges of the shapes, are they cut or moulded? In a more modern piece, simply find a patch where you can scuff the material without wrecking the item. If it scuffs grey/white....you've got a piece of plastic in your hand, if it scuffs brown, it's real. Jet is rarely faked using glass as glass is simply too heavy and the fakery would be too obvious. In older times, Jet was also used to make small drinking bowls used during a lady's confinement (birthing) time. Drinking water that had been in Jet was thought to relieve pain.