Ruby
Rubies and sapphires are naturally occurring minerals with definite chemical compositions, atomic structures and crystal structures. The crystal system of a ruby is a reflection of the internal arrangement of atoms, according to Dr. Susan Ward Aber of Emporia University. The crystal structure of a ruby is hexagonal (trigonal), dipyramidal. You can imagine the crystal system of a ruby by thinking of two pyramids stuck together at the bases.
The Crystal System
Hexoganal crystals grow as pyramids.
The crystal system is a method used to explain the atomic arrangement of the chemical components of minerals. There are six classifications: isometric, tetragonal, hexagonal, monoclinic, triclinic and orthorhombic. Rubies develop in a hexagonal crystal system and have four crystallographic axes. Three axes occur in one plane. They are all the same length and intersect at 120 degrees, says Dr. Ward Aber. The fourth axis is perpendicular to the plane of the other three and a different length. In other words, hexagonal crystals grow in the shape of pyramids or hexagonal prisms. Rubies form in the shape of dipyramids.
Dipyramids
Hexagonal crystals
The dipyramidal crystal form of a ruby is made of two pyramids placed base-to-base. According to physics professor David P. Belanger, each pyramid can have have three, four, six, eight or 12 faces. Therefore, the entire dipyramidal crystal of a ruby can possess a total of six, eight, 12, 16 or 24 faces.
Significance
Corundum is used to make sandpaper.
Rubies are corundum minerals. Corundum has a hardness of 9 on Mohs Scale. This strength is due to the crystal structure of these minerals. Diamond is the only known mineral that is harder than corundum. While the color of rubies makes them popular as gemstones, less colorful corundums are very useful as abrasives. The crystal structure and strength of corundum makes it important to industry. Crushed corundum is used to make grits and powders for polishing compounds, sandpaper, cutting tools and grinding wheels. As a colorful gemstone, the sturdy crystal structure of a ruby means it is so durable that it is nearly indestructible. The stones in ruby jewelry will withstand generations of everyday wear, making them excellent heirloom pieces.
Inclusions
A cat's eye ruby.
While the crystal structure of a ruby never deviates, inclusions of other minerals are common. Unlike most other gems, these impurities can actually increase the value of a ruby. Included rutile needles in a ruby cause a silky shine. These stones can be cut as a cabochon and will display a cat's eye effect, highly prized in jewelry. Rutile crystal inclusions can cause asterism, a six-rayed-starlight effect on a ruby cabochon. Note that included rubies cut to display these visual effects are incredibly beautiful, but they are not as durable as some other rubies.
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