Quartz (vein quartz)

Hardness: 7 (Mohs scale); Color: Typically white, milky white, greyish due to inclusions; Luster: Vitreous (glassy); Crystal Structure: Hexagonal (often massive or anhedral in veins); Cleavage: None (conchoidal fracture); Specific Gravity: 2.65

Quartz veins are formed when silica-rich fluids (often hydrothermal) circulate through cracks and fissures in existing rocks. As these fluids cool, the dissolved silica precipitates out, forming quartz crystals that fill the voids. This process can occur across a wide range of geological ages, from ancient Precambrian to relatively recent Cenozoic activity, often associated with tectonic events, metamorphism, and igneous intrusions.

Quartz, especially in its milky or massive form as seen in veins, has numerous industrial uses: as an abrasive, in glass manufacturing, as a flux in metallurgy, in electronic components (piezoelectric properties), and in construction as an aggregate. Clearer, more crystalline varieties are used in jewelry (e.g., amethyst, citrine, rock crystal).

Quartz is one of the most common minerals on Earth's continental crust, second only to feldspar. Its high resistance to weathering makes it a primary component of sand and sedimentary rocks like sandstone. Vein quartz can sometimes host valuable ore minerals like gold or silver, making it an important indicator for prospectors.

In the field, milk-white to greyish, vitreous rocks that are harder than steel (will scratch glass easily) and do not react to acid (unlike calcite) are likely quartz. It often appears as irregular masses or veins cutting through other rock types. It's ubiquitous globally, found in almost all geological settings from igneous to metamorphic to sedimentary rocks. Collectors often look for well-formed crystals within vugs or associated with other interesting minerals.