For example, 'oriental amethyst' is purple corundum, 'oriental topaz' is yellow corundum, 'oriental emerald' is green corundum, etc. Gemologists and gem dealers will often deceivingly use the term 'oriental' in referring to non-blue colored sapphires. Sapphire covers the largest number of colors. If transparent and relatively fracture-free & inclusion-free, corundum is said to be of gem-quality, and the color determines the name of the gem. The color of corundum is variable - it can be any color, including plaid patterns. Additional breakages will not be along planar surfaces. The cleavage-looking flat tops & bottoms of corundum are called partings (pseudocleavage). These flat ends are not cleavage planes - corundum has no cleavage. The hexagonal columns of corundum typically have well-developed flat tops & bottoms. Corundum forms hexagonal crystals, which is evident even in many river-worn specimens. At H≡9, it is the hardest common mineral, apart from diamond. Gold oxide forms only under special conditions.Ĭorundum is aluminum oxide - Al2O3. All except the most inert elements (such as the platinum-group elements and gold and noble gases) readily form oxides. In the presence of oxygen and moisture, sulfide minerals tend to tarnish or alter to oxides and hydroxy-oxides. Many sulfide minerals are not stable in Earth-surface conditions. The hydroxide minerals (those with one or more OH-) are usually considered together with the oxides. The oxide minerals include species that are hydroxy-oxides. The oxide minerals all contain one or more oxide anions (O-2).

Major categories of minerals are: elements, sulfides, oxides, halides, carbonates, sulfates, phosphates, and silicates. Mineral classification is based on anion chemistry. Currently, there are over 5500 named and described minerals - about 200 of them are common and about 20 of them are very common. At its simplest, a mineral is a naturally-occurring solid chemical. Blue corundum (sapphire) (~5.0 centimeters across at its widest)Ī mineral is a naturally-occurring, solid, inorganic, crystalline substance having a fairly definite chemical composition and having fairly definite physical properties.

Corundum blue