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What Type of Rock Is Sapphire?

One of the four precious gems, sapphire is a form of corundum. The most valuable sapphires range from a cornflower blue to a deep royal blue in color, but sapphire naturally occurs in almost every color except red. Red corundum has its own special category; jewelers refer to red corundum as ruby. Gem-quality sapphire appears in jewelry as faceted stones or as smooth cabochons, a rounded shape that showcases the gem's vivid color.

Igneous Rock
- All rocks fall into one of three broad categories: igneous, sedimentary and metamorphic. The first of these, igneous rocks, are the crystallized or solidified results of molten magma or lava. Corundum is an igneous rock, although it also occurs in metamorphic and sedimentary formations. Its extreme hardness -- sapphire is a 9 on the Mohs hardness scale -- makes it less susceptible to the weathering that erodes the rock around it. Pieces of corundum loosen from softer surrounding rock and wash into streams where it finds its way into future sedimentary rock deposits.
Corundum
- Geologists refer to crystalline aluminum oxide of any hue as corundum. Pure Al2O3 is colorless. Jewelers call this colorless mineral clear sapphire or white sapphire when it is transparent enough to shape into a gemstone. Impurities in the stone lead to other colors. Blue sapphire contains traces of iron and titanium; differing proportions of these impurities impart different shades of blue to the gem. Chromium gives sapphires a pink hue; more of this impurity turns corundum red, taking it out of the sapphire family and placing it with rubies.
Properties
- Corundum tolerates very high temperatures and pressures, making these gems well suited for tension-set mounting in jewelry. Corundum gems often form with inclusions of rutile, thin hairlike projections of titanium oxide. Small amounts of rutile give the gem a cloudy appearance that jewelers refer to as silk to describe the sheen the fibers impart. Sapphire exhibits pleochroism, or color changes that depend on the viewing angle. Jewelers cut sapphires with careful attention to their pleochroic character so that the finished gem reveals its most vivid hue when a viewer sees the stone directly from above.
Gem Quality
- While not all aluminum oxide crystals merit the term "gem," sapphires are a special case. Industrial-quality corundum functions as an abrasive and as a cutting tool, but although the abrasive grit on corundum sandpaper is chemically identical to sapphire, the two look little alike. Sapphire joins the ranks of other gems because its hexagonal crystalline structure, clarity and color make it beautiful when a jeweler facets or polishes it.

Rocks & Minerals