Agate

Hardness: 6.5-7 on Mohs scale; Color: typically banded with various shades of white, gray, blue, brown, or red; Luster: waxy to vitreous; Crystal structure: cryptocrystalline (microcrystalline quartz); Cleavage: none, conchoidal fracture; Specific gravity: 2.58-2.64. The specimen shown is a polished heart-shaped pendant exhibiting distinct light and dark gray/white banding, characteristic of lace or plume agate varieties.

Forms primarily in volcanic and metamorphic rocks. Silica-rich fluids percolate through cavities (vesicles) in the rock over thousands to millions of years, depositing layers of microscopic quartz crystals. The banding is caused by variations in the mineral content and impurities during deposition.

Predominantly used for lapidary arts, jewelry making, ornamental objects, and as collector specimens. Historically used for making precision items like pestles, mortars, and knife-edge bearings due to its hardness and resistance to chemical attack.

Agate is named after the Achates River (now the Dirillo River) in Sicily, where it was first discovered by the Greek philosopher Theophrastus in the 4th/3rd century BC. It is known for its incredible variety of patterns and colors.

Identified in the field by its distinct banding, waxy luster, and hardness (scratches glass). It often occurs as nodules in basalt or other volcanic rocks. Found worldwide, with prominent deposits in Brazil, Uruguay, Mexico, Madagascar, and the USA. Collectors look for unique banding patterns, vivid colors, and overall shape.