| Corundum is the second hardest natural mineral | | | | These stones are also known as ""sapphires"", |
| known to science. | | | | however, when the color of a sapphire is any |
| The History Says | | | | color other than deep blue the color of the stone |
| One century ago, the richest corundum mine in | | | | is always used as an preceding adjective. For |
| the world was in Canada, at Craigmont, located | | | | example: pink sapphire, yellow sapphire, green |
| several kilometres south of Combermere, Ontario, | | | | sapphire. Used alone, the word ""sapphire"" refers |
| in the woodsy hill country of Raglan and Carlow | | | | to the deep blue corundum. |
| Townships. | | | | The color of corundum can be artificially altered |
| Corundum is a very interesting mineral. It is an | | | | by heating or radiation. Sometimes cloudy or |
| exceptionally hard aluminum oxide that forms in | | | | translucent stones can be heated to yield brighter |
| igneous and metamorphic rocks. It is primarily | | | | color or more transparent stones. When these |
| used as an abrasive. Corundum is also one of the | | | | color treatments are done the stone is supposed |
| most popular minerals - rubies and sapphires are | | | | to be labeled as ""heat treated"" when it is |
| colored corundums. Rubies are deep red | | | | presented for sale. |
| corundums of gemstone quality. Gemstone quality | | | | Mining Rubies and Sapphires |
| corundums of any other color are sapphires. | | | | Most gem-grade corundums form in metamorphic |
| Hardness and Use as an Abrasive | | | | rocks such as schists or igneous rocks such as |
| Corundum is very hard. It is the standard mineral | | | | syenite. However, they are rarely mined from the |
| for a Mohs Scale hardness of nine. It is the | | | | rocks in which they form. Instead, they are |
| second hardest mineral known, with diamond | | | | usually mined from stream sediments. Rubies and |
| being the only harder mineral. | | | | sapphires are very hard and that enables them to |
| This high hardness makes corundum especially | | | | survive the abrasive actions of a stream. Their |
| useful as an abrasive. Crushed corundum is | | | | high specific gravity, relative to other sediment |
| screened to produce uniformly-sized grits and | | | | particles, often causes currents to concentrate |
| powders which are used as grinding media and to | | | | them in small deposits. Most rubies and sapphires |
| manufacture polishing compounds, sand papers, | | | | are produced by washing the gravels of these |
| grinding wheels and cutting tools. | | | | stream deposits. |
| Emery | | | | Corundum Occurrence |
| Emery is a granular metamorphic or igneous rock | | | | Corundum is found in igneous rocks such as |
| that is rich in corundum. It is a mixture of oxide | | | | syenite, nepheline syenite and pegmatite. It is |
| minerals, typically corundum, magnetite, spinel and | | | | found in the rocks of contact metamorphism and |
| or hematite. It is the most common form of | | | | in mica schist, gneiss and marble. Because it is |
| natural corundum used to manufacture abrasives. | | | | very hard and chemically resistant, corundum is |
| Red Corundum: Ruby | | | | also found in alluvial deposits. |
| Most corundum occurs as white, gray, dull blue or | | | | Synthetic Corundums |
| dull red crystals. However, a very small amount | | | | Natural deposits of corundum are often impure |
| of corundum has a transparency, purity and color | | | | and not available in large quantities where it is |
| that make it suitable for use as a gemstone. | | | | needed for manufacturing abrasives. This need is |
| These colorful corundums are among the most | | | | often met by producing synthetic corundum from |
| popular and valuable gems. | | | | bauxite. The synthetic corundum is of high quality |
| Some gem-quality corundum is contaminated with | | | | and cost competitive with natural sources. |
| trace amounts of chromium. A very small amount | | | | Synthetic rubies and sapphires are also produced. |
| of chromium gives corundum a pink color and | | | | The manufacturing process can produce large, |
| larger amounts produces stones that are deep | | | | flawless single crystals which can be cut into |
| red. These deep red corundums are known as | | | | attractive gemstones. The color in these stones |
| ""rubies"". Transparent rubies are cut into faceted | | | | can be controlled by adding small amounts of |
| stones and translucent rubies are cut into | | | | chromium, titanium or other substance. Special |
| cabochon-shaped gemstones. | | | | optical effects such as asterism can be imparted |
| Blue Corundum: Sapphire | | | | to the stones by adding titanium or another |
| Corundum that is contaminated with iron or | | | | material which crystallizes in the form of needles |
| titanium has a blue color. Deep blue corundums | | | | (such as rutile). These needles can be in |
| are known as ""sapphires"". When used alone, the | | | | alignments which produce a ""star"" appearance |
| word ""sapphire"" always refers to a deep blue | | | | when the stones are cut and polished. The |
| corundum. | | | | synthetic stones are to be labeled when sold and |
| Gem quality corundum occurs in a wide range of | | | | it is often difficult for even an expert to |
| colors, including pink, pale blue, yellow and green. | | | | distinguish them from natural stones. |