Garnet

Learn about garnet, a group of silicate minerals with many varieties.

Overview

Garnet is actually a group of closely related silicate minerals that share the same crystal structure but have different chemical compositions. Garnets come in many colors and are valued as gemstones and for their use in industry.

What Is Garnet?

Definition: Garnet is a group of silicate minerals that share the same crystal structure (cubic) but have different chemical compositions. The name "garnet" comes from the Latin "granatum" meaning pomegranate, referring to the red color of some varieties. Composition: General formula: X₃Y₂(SiO₄)₃ where: - X = Ca, Mg, Fe²⁺, Mn²⁺ - Y = Al, Fe³⁺, Cr³⁺ Common Varieties: - Almandine: Red to purple (Fe₃Al₂Si₃O₁₂) - Pyrope: Red to purple (Mg₃Al₂Si₃O₁₂) - Spessartine: Orange to red (Mn₃Al₂Si₃O₁₂) - Grossular: Many colors (Ca₃Al₂Si₃O₁₂) - Andradite: Yellow, green, brown (Ca₃Fe₂Si₃O₁₂) - Uvarovite: Green (Ca₃Cr₂Si₃O₁₂) Color: Can be almost any color - red, orange, yellow, green, brown, black, colorless.

Formation and Occurrence

How It Forms: Garnet forms in many different environments: - Metamorphic: Common in schist, gneiss, and other metamorphic rocks - Igneous: Some igneous rocks, especially pegmatites - Contact Metamorphism: In skarn deposits - Alluvial: Can be found in river and beach deposits Where It's Found: - Metamorphic rocks worldwide - Some igneous rocks - Gem deposits - Industrial deposits Geological Settings: - Regional metamorphism - Contact metamorphism - Pegmatites - Some igneous intrusions

Physical Properties

Hardness: 6.5-7.5 on Mohs scale (hard, durable) Crystal System: Cubic (isometric) Crystal Form: Usually forms dodecahedrons (12-sided) or trapezohedrons (24-sided) Luster: Vitreous to resinous Fracture: Conchoidal to uneven Uses: - Gemstones (many varieties) - Abrasive (industrial use) - Waterjet cutting - Some varieties are very valuable (demantoid, tsavorite)

Gem Varieties

Almandine: Deep red, most common gem garnet Pyrope: Red to purple, often called "rhodolite" when purplish-red Spessartine: Orange to red, sometimes called "mandarin garnet" Grossular: Many colors: - Tsavorite: Green (very valuable) - Hessonite: Orange to brown - Rhodolite: Pink to purple Andradite: - Demantoid: Green (very valuable, high dispersion) - Topazolite: Yellow Uvarovite: Emerald green (rare, usually small crystals)

Rockhounding Value

Collecting Considerations: - Many beautiful colors - Well-formed crystals are common - Can be found in many rock types - Some varieties are very valuable Where to Look: - Metamorphic rocks (schist, gneiss) - Pegmatites - Skarn deposits - Alluvial deposits - Areas known for garnet What to Collect: - Well-formed crystals - Colorful varieties - Large crystals - Gem-quality specimens - Specimens in matrix