Blue corundum (sapphire) from Sri Lanka. (~1.65 centimeters tall)

A mineral is a naturally-occurring, solid, inorganic, crystalline substance having a fairly definite chemical composition and having fairly definite physical properties. At its simplest, a mineral is a naturally-occurring solid chemical. Currently, there are over 5400 named and described minerals - about 200 of them are common and about 20 of them are very common. Mineral classification is based on anion chemistry. Major categories of minerals are: elements, sulfides, oxides, halides, carbonates, sulfates, phosphates, and silicates.

The oxide minerals all contain one or more oxide anions (O-2). The oxide minerals include species that are hydroxy-oxides. The hydroxide minerals (those with one or more OH-) are usually considered together with the oxides. Many sulfide minerals are not stable in Earth-surface conditions. In the presence of oxygen and moisture, sulfide minerals tend to tarnish or alter to oxides and hydroxy-oxides. All except the most inert elements (such as the platinum-group elements and gold and noble gases) readily form oxides. Gold oxide forms only under special conditions.

Corundum is aluminum oxide - Al2O3. At H≡9, it is the hardest common mineral, apart from diamond. Corundum forms hexagonal crystals, which is evident even in many river-worn specimens. The hexagonal columns of corundum typically have well-developed flat tops & bottoms. These flat ends are not cleavage planes - corundum has no cleavage. The cleavage-looking flat tops & bottoms of corundum are called partings (pseudocleavage). Additional breakages will not be along planar surfaces.

The color of corundum is variable - it can be any color, including plaid patterns. 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.

deep red = ruby blue = sapphire pale red = sapphire pale green = sapphire purple = sapphire yellow = sapphire

Sapphire covers the largest number of colors. Gemologists and gem dealers will often deceivingly use the term "oriental" in referring to non-blue colored sapphires. For example, "oriental amethyst" is purple corundum, "oriental topaz" is yellow corundum, "oriental emerald" is green corundum, etc. Black-colored corundum is often called emery. Corundum-rich rocks are also called emery, or corundite (<a href="https://www.flickr.com/photos/jsjgeology/albums/72157646922258716">www.flickr.com/photos/jsjgeology/albums/72157646922258716</a>).

The cornflower blue-colored sapphire crystal shown above is from the famous gem gravels of Ceylon, off the southern tip of India.

Locality: "gem gravels", unrecorded/undisclosed mine attributed to the Ratnapura area, Ratnapura District, Sabaragamuwa Province, southern Ceylon (Sri Lanka)

See locality info. at: <a href="https://www.mindat.org/loc-3147.html" rel="noreferrer nofollow">www.mindat.org/loc-3147.html</a>

Photo gallery of corundum and sapphire: <a href="http://www.mindat.org/gallery.php?min=1136" rel="noreferrer nofollow">www.mindat.org/gallery.php?min=1136</a> and