Garnets form an isomorphous series of silicate minerals with the general formula X₃Y₂(SiO₄)₃. Common species include almandine (red-purple), pyrope (red), spessartine (orange), grossular (green tsavorite or hessonite), and andradite (demantoid green). High refractive index (often 1.7–1.9) produces strong brilliance.
Garnets lack cleavage and rate 6.5–7.5 in hardness. They occur in both metamorphic and igneous rocks and concentrate in alluvial deposits due to durability. Color is caused by iron, manganese, chromium, or vanadium. Some show strong dispersion or color-change effects.