Quartz: The family name for crystals composed of silica or silicon dioxide occurring in hexagonal crystals. The most common variety is colorless and transparent. This is often referred to as Clear Quartz, Rock Crystal or simply Quartz. The crystalline varieties include: amethyst, ametrine, citrine, rose quartz, and smoky quartz. There are a variety of crypto-crystalline varieties including agate (chalcedony), onyx, sardonyx, carnelian, sard, chrysoprase, bloodstone, jasper, and prase. (see individual listings). Quartz is an essential constituent of granite, forms the rocks quartzite and , and makes most of the sand of the seashore.