CorundumCorundum is best known for its gem varieties, Ruby and Sapphire. Ruby and Sapphire are scientifically the same mineral but just different colors. Ruby is the red variety, and Sapphire is the variety that encompasses all other colors, although the most popular and valued color of Sapphire is blue. Sapphire is also only used to describe the gem variety, otherwise it is simply called Corundum.
Corundum is a very hard, tough, and stable mineral. For all practical purposes, it is the hardest mineral after Diamond, making it the second hardest mineral. It is also unaffected by acids and most environments. Translucent brown Corundum and Emery are the most common forms of Corundum. These are fairly common forms, and due to their great hardness and prevalence are the most favorable abrasives. The industrial term "emery" describing Corundum abrasives is derived from the variety Emery which is mined specifically for its use as an abrasive. Erosion may cause Emery to crumble and form sand, which may be called "black sands". http://www.minerals.net/mineral/corundum.aspx HematiteHematite, also spelled as haematite, is the mineral form of iron(III) oxide (Fe2O3), one of several iron oxides. Hematite crystallizes in the rhombohedral system, and it has the same crystal structure as ilmenite and corundum.
Hematite is a mineral, colored black to steel or silver-gray, brown to reddish brown, or red. It is mined as the main ore of iron. Varieties include kidney ore, martite (pseudomorphs after magnetite), iron rose andspecularite (specular hematite). While the forms of hematite vary, they all have a rust-red streak. Hematite is harder than pure iron, but much more brittle. http://www.minerals.net/mineral/hematite.aspx MagnetiteMagnetite is best known for its property that it is strongly attracted to magnets. Some forms of Magnetite from specific localities are in fact themselves magnets. This form is known as Lodestone, and is the only mineral that acts as a natural magnet. Although its magnetism is weak, it is strong enough to attract large nails. Due to its magnetic properties, small iron particles are often found clinging to its surfaces. Some dealers intentionally place metallic filings on a Lodestone to demonstrate its magnetism.
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