Quartz is one of the most common minerals found in the Earth’s crust. This mineral group offers a rich range of gem varieties that can be used in jewellery. Quartz comes in two forms: single-crystal quartz and polycrystalline quartz. The former includes amethyst, ametrine, citrine, prasiolite, rose quartz and smoky quartz amongst others. The polycrystalline quartz varieties are aggregates and called chalcedony (aquaprase, carnelian, chrysoprase, onyx). When banded in texture they are termed agates. Other forms of polycrystalline quartz include jasper and silicified woods and bones.