Garnet Mineral Characteristics

If pure and flawless, garnet can be transformed from mineral to gem.

While the name "garnet" may bring to mind a luminous, deep red gemstone, the mineral form of garnet is actually quite rocky, sharp and hard, making it a perfect ingredient for abrasives. Otherwise, if the mineral garnet is pure and flawless, it becomes transformed and cut into its more commonly recognized gem.

Chemical Properties

Garnet is a combination of silica minerals which crystallize into isometric, or symmetrical shapes such as cubes or hexagon. Most garnets are six- or eight-sided crystals, although chemical properties vary with different forms of the mineral. All garnets contain the silicate molecule, but different varieties also contain specific metal ions, such as iron, aluminum, chromium and magnesium. In its mineral form, garnet can be colorless, red, pink, yellow or dark brown.

Hardiness

When broken, a segment of garnet fractures into smaller, sharper pieces, an element of its highly abrasive properties. On the Mohs' hardness scale, garnets range from 6 to 7.5, which is quite hard for a mineral. In comparison, a diamond's hardiness is a 10 on the Mohs' scale, while quartz and topaz are respectively ranked at 7 and 8. Garnet has a low cleavage when compared to other minerals. Cleavage is measured by the mineral's ability to break into smaller pieces when touched, with mica a common example for strong cleavage. The low-cleavage properties of garnet make it exceptionally useful as an abrasive.

Light Properties

Garnets are ranked as a 1.8 on the refractive index, which measures how light passes through the gem and is reflected, or bent, through the stone. Garnets are also considered isotropic, which means the mineral remains dark when cross-polarized light crosses the surface.

Usage

Because of this hard-edged quality, garnet minerals are used as an ingredient in abrasive powders and blasting materials, sanding strips, belts and discs, and as component of water filtration. Garnet is mined for industrial use in Idaho, New York and Montana, as well Australia, China, India, Russia and Turkey.

References

• Mineral Information Institute: Garnet
• North Carolina State University: Useful Mineral Differentiation Criteria

Resources

• International Colored Gemstone Association: Garnets
• Enchanted Learning: Rock and Mineral Dictionary