Glass as a gem
It might seem odd to include glass in a list of gems, but glass has been one of the most common materials used in jewellery for more than 3000 years, both in its own right or as imitations of natural gems. Remember, there is nothing illegal or underhand about imitation gems, just as long as the person buying them knows what they are buying.
Properties of glass
If you look carefully at glass, you may see bubbles and swirls that are typical of it, but they may not always be present. Remember, some inclusions in natural gems can look very much like gas bubbles.
Most of the glass used for imitation gems (often called 'paste') is quite soft at about 5 to 5.5 on Mohs' scale, so glass gems will often be scratched or have worn facet edges, things that can be easily spotted with a 10x loupe. Glass will never show different colours with a dichroscope as it is not pleochroic.