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Silicates

All silicates have formula SiO₂. Opal also has some extra water in its molecule. The silicates are the commonest stones that you will find being sold in rock shops. They are also used as semi-precious stones in jewelry. This is not surprising, since they are often beautifully coloured or patterned. The quartz have magnificent crystals. They are also hard stones, so don't crumble or break easily. Also the crystals and polished stones have a hard shine to the surface. The silicates that can form crystals are called quartz, although some have other names as well, so purple quartz is amethyst. The silicates that can't form crystals are chalcedony and agate. Chalcedony does not have stripes and agate does, although there are many different names for the different colours and patterns.