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What Are Rubies Made Out of?

Rubies share most of their chemical composition with another precious gem, the sapphire. Both are corundum, but rubies' characteristic velvety red looks like no other gemstone. Rubies and sapphires both form in igneous rock, but rubies require chromium to attain their brilliant red hue. Ruby is one of the hardest gemstones, on par with sapphire and below only diamond on the Mohs hardness scale.

Chemical Structure
- Wearing a ring of crystalline aluminum oxide with chromium impurities doesn't sound very alluring, but it defines a ruby precisely. Chromium ions replace some of the aluminum ions within the crystalline lattice; these ions change how the gem absorbs and reflects different wavelengths of light. The gem absorbs yellow and green light while reflecting the remainder of the light as a luminous red. Differing amounts of chromium account for rubies' varying shades of red.
Color
- Color is what separates a ruby from other forms of corundum. However, gemologists disagree on where the line between pink sapphire and pale ruby should lie. One jeweler's deep fuchsia sapphire is another's light rosy ruby. Until the early 20th century, pink sapphires fell under the ruby rubric; today, jewelers consider them a less valuable gem in their own right. Jewelers and buyers agree, though, that the most valuable rubies are a luminous blood red.
Inclusions and Asterism
- Natural rubies of perfect clarity are rare enough to drive their price beyond the reach of all but the most wealthy buyers. However, a ruby with "silk" -- thread-like parallel rutile inclusions that give the stone a silky sheen -- proclaims its natural origins without detracting significantly from the gem's beauty. Silk makes a ruby translucent instead of transparent and can even improve its color as light scatters within the stone instead of traveling through it. A ruby with especially abundant silk becomes opaque, bouncing direct light back to the viewer in the form of a six-rayed star. Gemologists call this phenomenon asterism. Natural star rubies command high prices.
Synthetic Corundum
- Synthetic rubies are chemically identical to natural rubies.
  
  Laboratory-created rubies still contain the same aluminum oxide and chromium formula as natural rubies, but with none of a natural ruby's imperfections. Despite their superficial perfection, though, synthetic rubies fetch considerably lower prices than all but the smallest and cloudiest natural rubies. Their easy availability and perfection work against them; gem collectors and jewelers appreciate a natural ruby's "personality," the flaws and inclusions that make it unique.

Rocks & Minerals