Most of the rocks in this selection of tumbled or polished are either quartz or a quartz variation. The clear variety of quartz is often called rock crystal. But other varieties, depending on color and mineral inclusions, are amethyst, adventurine, Eisenkiesel, rose quartz, Tiger eye, Cat's eye, Jasper, Flint, Heliotrope, plasma, chalcedony, agate, carnelian and chrysoprase. See Types of Quartz. Quartz is made up of fused sand or silicon dioxide, SiO2. Quartz rates 7 on the Mohs Scale of hardness where diamond is rated as 10.
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