There are a number of stones that are used to copy other gems, like diamonds. For instance, there are quite a few natural colourless gemstones around that have been used as a diamond copy. They include spinels, sapphires, zircons, quartz, goshenite (Beryl) and topaz. There are also synthetic stones like cubic zirconia and moissanite.
The pretty early Victorian pendant above is set with garnets and colourless spinels. The Georgian earrings below are set with foiled colourless quartz, with some discolouration of the foiled backs.
Glass, though, has consistently being used to copy diamonds. Of the three rings in the photo at the top of the post, two, which are Georgian, are paste and one, the Victorian bottom half hoop, is set with diamonds. It can be hard to differentiate between good quality paste and diamonds, particularly as they may both be set in gold.
Have a look at the two earrings below, one is set with glass paste, the other is set with diamonds. Both are made of 18ct gold but the one on the left has a silver front. One is Georgian, with closed backs to the diamonds, the other is a Georgian reproduction, also with closed backs. The settings of the stones with the closed collets make is difficult to really examine the stones to identify them.
The Georgian earring is on the left and is set with diamonds, while the reproduction earring on the right is set with paste glass.
navette jewellery 