How to know if a crystal is real or it's just plain glass?

I suppose it would be that if it looks too perfect, it is an imitation. I know that diamonds scratch, while imitations do not. Semi-precious stones have flaws because they are formed naturally while fakes are made by man and they have no flaws. So if the stone has a flaw, it is real, but if it does not, it is a fake. It is better to ask a jeweler if your stone is real or imitation. He should tell you.

A simple test is to test the crystal's scratching hardness (also known as Mohs hardness). All you need is a piece of glass and the crystal (it's only possible if the crystal has sharp edges, or tips). You try to scratch the glass with the crystal. Semiprecious and precious stones have a hardness of over 6 on Mohs's scale (because they are varieties of quartz, or corundum, topaz and diamond), and they scratch the glass. If you have a fake, made of glass or plastic, it won't scratch the glass. This test only helps if you're talking about the stones mentioned above, in brackets. There are other semiprecious stones (such as malachite, hematite, etc.), basically metal salts, that have a lower hardness than glass. For those you can make another test: the true color test. You need a piece of white ceramic, but with no email on it. You rub the crystal on the piece of ceramic and it leaves a trace, sometimes of a different color than the actual crystal. That's called the true color, or the powder's color. Glass or plastic don't have another color, while, for example, hematite, which can be from red to black, leaves a dark red trace.