When studying a gemmology diploma, students are required to complete written assignments and exams as well as practical exams where you need to identify a variety of gemstones. The hardest stone that I was given to identify during my course was a black gemstone. The reason why it is so hard to identify what a black stone is, is that many of the tests that gemmologists do when trying to identify a stone depend on being able to see inside the stone (for inclusions) and to see how the stone interacts with light, using a variety of tools (polariscope, dichroscope, refractive index, and spectroscope). The colour is also important. With a black stone, these tests are generally not possible. If the stone is unset, then a specific gravity test can be conducted but if the stone is set, then all that may be left as testing tools are UV light and the hand lens.
Surprisingly, there are quite a few black gemstones. You can find the following black gems – spinel, garnet, tourmaline, diopside, nephrite, onyx, obsidian, cubic zirconia, black coral and opal, as well as jet. The gemstones above are star diopside. There is also black glass, which in antique jewellery was usually French jet (used in the 18th and 19th centuries as a replacement for Whitby jet), Vauxhall glass and paste.
One important differentiator with these black stones is their hardness. By examining the surface of the stones, you can get a good idea about their identity by the wear that can be seen. Jet for instance is a soft stone, only about 2 ½ to 4 on the Mohs hardness scale, while paste and glass rate at about 6 on the scale. Black garnets are between 6 ½ to 7 ½, onyx is at 7, black tourmaline at 7 to 7 ½, black spinel at 8, black sapphire at 9 and black diamonds at 10.
The little lace pin above is set with jet and you can see how worn it is. The lace pin below, however, is set with black garnets.
Sometimes the lustre of a gemstone can help with identification. Lustre is the light that is reflected from the surface of a gemstone. Most gemstones and glass have a vitreous lustre. However, even those, like jet, which has a waxy lustre, can be polished to a shiny vitreous finish.