Quartz is a hard crystalline mineral composed of silicic acid (silicon dioxide). The atoms are connected in a continuum of silicon-oxygen-SiO4 tetrahedra, with each oxygen shared between two tetrahedra, giving a general chemical formula for SiO2. Quartz is the second most abundant mineral in continental crust after feldspar. Quartz exists in two forms, regular α-quartz and high-temperature β-quartz, both of which are chiral. The transformation from α-quartz to β-quartz occurs abruptly at 573 °C (846 K; 1063 °F). Since the transformation is accompanied by a significant change in volume, micro-fractures of ceramics or rocks exceeding this temperature threshold can easily occur. There are many types of quartz, some of which are graded gemstones. Since ancient times, varieties of quartz have been the minerals most commonly used to make hardstone jewelry and sculptures, particularly in Eurasia. Quartz is the mineral that defines a value of 7 on the Mohs scale of hardness, a qualitative scratching method used to determine a material’s resistance to abrasion.