As far as jet imitations go, French jet is one of the most common. For a collector, it’s hard to discern directly a piece of French jet, not because it’s to be confused with real jet in any way, but simply because many of the designs were so innocuous that finding a 19th century piece of French jet and identifying it from a piece of black glass used all the way through to the 1940s can be difficult. When French jet designs are used in more period styles, such as hat pins or showing more typical bold 19th century designs, then it’s much simpler, but there is an absolute abundance of French jet on the market that have been torn from buttons, trimming and various ancillary accessories.
English variations of French jet are called Vauxhall glass and you can often spot a piece of either French jet or Vauxhall glass from its cold touch, high reflective surface (strangely enough, like glass) and it will have a touch of red when held on some angles towards the light.
Quite often found in the trim to mourning dresses (particularly in the second and third stages due to its reflective surface and light weight), French jet was heavily produced and very cheap.