 Zircon
crystallizes in the tetragonal systemi.
Its hardness is about 6.5 - 7.5, the chemical composition is zirconium-silicate.
Due to a high concentration of uranium and thorium, zircon is radioactive. That leads to a slow breakdown of the crystal lattice  and changing optical
characteristics such as refraction and birefrigenge.
Therefore zircons are devided into high and low types.
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The
colours of zircon: yellow, brown, violet, blue, green and red or evem colourless.