It’s hard to imagine that a mineral with a name as mundane as corundum yields gems as exquisite as the ruby and sapphire, or even that these two stones, so different in color and mystique, are actually the same mineral family.
Lucky you if your birthstone is sapphire (September) or ruby (July). These are among the richest-colored of all gemstones with a romance and history as colorful as they are. Rubies are actually rarer than sapphires, and only red corundums are called rubies. Any other color is a sapphire. When grading colored stones, the density and hue of the color are part of the evaluation, and it’s the richest, deepest colors that are the most prized. In rubies, the most prized variant of color is called pigeon’s blood. Large gem quality rubies can be more valuable than comparably sized diamonds and are certainly rarer. There is a relative abundance of smaller, (1-3 carat,) blue sapphires compared to the scarcity of even small gem quality rubies, making even these smaller stones relatively high in value.
Stones of Burmese origin generally command the highest prices. The vast majority of rubies are "native cut" in the country of origin. High value ruby rough is tightly controlled and rarely makes its way to custom cutters. Occasionally, such native stones are recut to custom proportions, albeit at a loss of weight and diameter. Custom cut and recut stones are usually more per carat.
Sapphires exist in all the shades of blue from the deep blue of evening skies to the bright and deep blue of a clear and beautiful summer sky. Sapphires also come in many other colors, not only in the transparent grayish misty blue of far horizons, but also displaying the bright fireworks of sunset colors – yellow, pink, orange and purple. So sapphires are really and truly heavenly stones, although they are being found in the hard soil of our so-called "blue planet”.
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Corundum is the form of mineral from which sapphires and rubies are derived. The mineral is naturally clear, but can have different colors when impurities are present. Transparent corundums are used as gems, and if they’re red, then they’re rubies, while all other colors are called sapphire. The word corundum comes from the Tamil kurundam Due to corundum's hardness (typically 9.0) it is commonly used as an abrasive in machining, from huge machines to sandpaper. Emery is an impure and less abrasive variety, with a Mohs hardness of 8.0. You’ve probably used corundum hundreds of times when you’ve filed your nails with an emery board!
Corundum is actually the second hardest substance on the earth, after the diamond, and as such has many industrial uses. Corundum is crushed, ground and screened to varying grain sizes. The grains are utilized in the manufacture of grinding wheels for flour and rice mills, and workshops. Grinding wheels are manufactured either by a mixture of corundum, clay and feldspar or by using sodium silicate as a bonding agent. Finer grains are utilized for the preparation of grinding-pastes for the automobile industry. Corundum paper and cloth finished to belts, discs, rolls, sheets and other shapes are prepared by giving a suitable coating generally of sodium silicate.
Due to its hardness corundum also finds use in mortars, wire drawing dies, thread guides and gauge blocks. Gem varieties are sometimes used for pivot supporters in delicate scientific instruments, as jewel bearing in watches.
Corundum in small quantities is used in the manufacture of special sparking plugs, mufflers, pyrometer tubes, rods and insulators for vacuum tubes of all kinds. Finely ground corundum, passing through 200 mesh, is used for the manufacture of the above products by giving them a bond of clay.
Diamond Related Articles
- American Topaz
- Corundum
- Diamond 4Cs
- Diamond Durability
- Diamond Mines
- Diamonds As Electrical Insulators
- Gemstones are Mineral Crystals
- How Diamond Prices Are Determined
- How Diamonds Are Made
- How Diamonds Are Mined
- How to Buy Diamond Engagement Rings
- How to Care for Your Diamond
- How to Clean your Diamonds
- How to Sell A Diamond
- How to Spot A Fake Diamond
- Industrial vs Gem Quality Diamonds
- Insuring Your Diamonds
- Ruby and Sapphire Gems
- Synthetic Diamonds
- Victoria Transvaal Diamond