Quartzite

Hardness: 7 (Mohs scale), Color: tan, yellow, white, gray, Luster: vitreous to grainy, Crystal Structure: Trigonal (microcrystalline), Cleavage: None (conchoidal fracture), Specific Gravity: 2.6-2.7

Formed from the metamorphism of quartz sandstone under intense heat and pressure, typically during regional mountain-building events (orogeny). Its age can range from Precambrian to Mesozoic depending on the source formation.

Used in construction as crushed stone or road ballast, as a dimension stone for flooring and wall coverings, and in the manufacturing of glass and ceramics when pure.

Quartzite is so hard and chemically resistant that it often forms the resistive 'cap' on top of hills or mountains, as it outlasts the softer rocks around it through erosion.

Check for a grainy, crystalline texture that does not scratch with a sapphire and can scratch glass easily. Unlike sandstone, quartzite breaks across the sand grains rather than around them. Found globally in mountainous regions.