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Instructions
Hardness
Apply a hardness test to determine the density of your stone. A natural ruby is rated at a 9.0 on the Mohs scale, meaning that rubies are very hard. The only stone harder than a natural ruby is a diamond, which rates a 10 on the same scale. Rubies can be damaged, however, it's unlikely that a natural ruby will be scratched easily. A simple test can be conducted using a mineral testing kit that can be purchased from many online retailers for as low as $10.
Rub the ruby over the surface of several mineral testing bars, starting at seven. A true ruby should be able to scratch all testing bars except for number 10. If the stone cannot scratch above bar six, you could be looking at glass. If the stone cannot scratch above eight, you may be looking at heat-treated topaz.
Remember to rub the stone over the mineral testing rod, not the other way around because you want to scratch the rod, not the stone.
Hold up the ruby to a bright light and examine it closely. A common flaw in synthetic rubies is the appearance of gas bubbles. These are caused by excessive oxygen produced during the fusion process. Some may be small enough to be undetectable to the naked eye, but larger visible bubbles can also form. While natural rubies will occasionally contain these inclusions, it is extremely rare.