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Rubies can command the highest per-carat price of any colored stone. Ruby is the birthstone for July and the gem for the 15th and 40th anniversaries.
Ruby is a variety of the mineral corundum, other varieties of gem-quality corundum are called sapphires. When it is pure, the mineral corundum is colorless, trace elements of chromium is what causes ruby’s red color. The more chromium, the stronger the color. Chromium can also cause fluorescence which will add strength to the color. All natural rubies have imperfections in them, including color impurities and inclusions of rutile needles known as "silk". Gemologists use these needle inclusions to distinguish them from synthetics or substitutes. Usually, the rough stone is heated before cutting, almost all rubies are treated in some form, with heat treatment being the most common practice. Untreated rubies of high quality command a large premium. Imitations go back to Roman times, early in the 17th century techniques were developed to color foil red, which was then placed under the imitation stone. They are also commonly glass-filled to fill in imperfections in the stone.
Rubies are evaluated on the basis of their geographic origin as well as the standard cut, clarity, carat weight and color. Upper Myanmar (Burma) was for centuries the world's main source for rubies. Central Myanmar began producing rubies during the 1990s and rapidly became the world's main ruby mining area. Rubies are also mined in Afghanistan and Pakistan. A few rubies have been found in the U.S. states of Montana, North Carolina, South Carolina and Wyoming.
The world's most valuable ruby is the Sunrise Ruby. On May 12, 2015, the 25.59-carat ruby ring sold for $1,266,901 per carat, setting a new record at auction for a colored gemstone. It has the sought after "pigeon blood" color, the most valuable color a ruby can have.
Ruby beads make a beautiful choker. Loose rubies will make future pieces.