Description

Corundum is best known for its gem varieties, Ruby and Sapphire. Ruby and Sapphire are scientifically the same mineral, but just differ in color. 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 stablemineral. 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 are sometimes called “black sands.”

Corundum is a crystalline form of aluminium oxide(Al2O3) typically containing traces of iron, titanium, vanadium and chromium. It is a rock-forming mineral. It is a naturally transparent material, but can have different colors when impurities are present. These impurities are the presence of transition metals in the crystal structure of corundum. Corundum has two primary gem varieties, ruby and sapphire. Rubies are red due to the presence of chromium, and sapphires exhibit a range of colors depending on what transition metal is present. A rare type of sapphire, padparadscha sapphire, is pink-orange.

The name “corundum” is derived from the Tamil word Kurundam, or the Sanskrit word Kuruvinda.

Because of corundum’s hardness (pure corundum is defined to have 9.0 on the Mohs scale), it can scratch almost every other mineral. It is commonly used as an abrasive on everything from sandpaper to large machines used in machining metals, plastics, and wood. Some emery is a mix of corundum and other substances, and the mix is less abrasive, with an average Mohs hardness of 8.0.

In addition to its hardness, corundum is unusual for its density of 4.02 g/cm3, which is very high for a transparent mineral composed of the low-atomic mass elements aluminium and oxygen.

Corundum crystallizes with trigonal symmetry in the space group R3c and has the lattice parameters a = 4.75 Å and c = 12.982 Å at standard conditions. The unit cell contains six formula units.

The toughness of corundum is sensitive to surface roughness and crystallographic orientation. It may be 6-7 MPa·m1/2 for synthetic crystals and ~4 for natural.

In the lattice of corundum, the oxygen atoms form a slightly distorted hexagonal close packing, in which two-thirds of the gaps between the octahedra are occupied by aluminum ions.

Corundum occurs as a mineral in mica schist, gneiss, and some marbles in metamorphic terranes. It also occurs in low silica igneous syenite and nepheline syenite intrusives. Other occurrences are as masses adjacent to ultramafic intrusives, associated with lamprophyre dikes and as large crystals in pegmatites. It commonly occurs as a detritalmineral in stream and beach sands because of its hardness and resistance to weathering. The largest documented single crystal of corundum measured about 65×40×40 cm (26×16×16 in), and weighed 152 kg (335 lb). The record has since been surpassed by certain synthetic boules.

Corundum for abrasives is mined in Zimbabwe, Russia, Sri Lanka, and India. Historically it was mined from deposits associated with dunites in North Carolina, US and from a nepheline syenite in Craigmont, Ontario. Emery-grade corundum is found on the Greek island of Naxos and near Peekskill, New York, US. Abrasive corundum is synthetically manufactured from bauxite. Four corundum axes dating back to 2500 BCE from the Liangzhou culture have been discovered in China.

Pure corundum is colourless and clear if transparent or pale white if opaque. This mineral also has low dispersion so the value of the stones comes not from fire generated (as in diamond), but rather from the intensity of colours seen. The vivid colours of corundum gem varieties, such as ruby and sapphire, arise primarily from elemental substitution in the Al site by transition metal elements. The most common cations to substitute are Fe+2, Fe+3, Ti+4, Cr+3, and V+3.

A continuum of colour saturation exists between pink sapphire and ruby that is correlated with trace amounts of Cr. There is no official cutoff for the amount of Cr needed for ruby, but usually rubies will have up to ~1 wt% of Cr2O3. When Cr substitutes for Al, wide absorption bands are generated in the violet (~450 nm) and green-yellow (~500 nm) ranges, as well as overlap a bit into the blue region. The red region of the electromagnetic spectrum (~650 nm) does not have very much absorption at all and results in all colours but red being blocked by ruby.

But there is another trick up ruby’s sleeve that makes its red almost jump out at the observer. When Cr is introduced into corundum it makes the mineral fluorescent under UV light. This means that UV energy from normal light is accepted into the crystal and then re-emitted at a lower energy level – conveniently in the red region, thus amplifying the intensity of red in ruby under daylight conditions. However, if any iron is present it will usually absorb the red fluorescence from UV light. Thus, the finest rubies are those that have little to no iron in their crystal structure.

Blue sapphires are generated primarily from pairs of Fe+2 and Ti+4 substituting into the crystal structure for Al+3. The process of intervalence charge transfer (essentially continual swapping of electrons, bouncing back and forth) occurs between the Fe and Ti and all colours except blue are absorbed. So like ruby, it is the absorption of all other colours from full spectrum light (aka white light) that generates the beautiful blues in sapphires, rather than the “generation” of the blue colour. Very small amounts of these elements (only ~0.01 wt% Fe and Ti) are needed to produce the vivid blues.

Other colours are generated from a combination of these elements, as well as other minor cations and defects in the crystals. Also, a single corundum gemstone can be multi-coloured from different concentrations of metals in different parts of the crystal – this is called zoning.

Some sapphires also show an optical characteristic called asterism, which is most commonly seen as a six or twelve pointed star. These “arms” of the star are generated from oriented inclusions of long and skinny minerals (almost always the mineral rutile, a titanium oxide, TiO2). Specimens found with these inclusions are often cut and polished in a rounded and polished cabochon style to emphasize the nature of this optical effect. Rutile inclusions can occur in both sapphires and rubies, although it is more common in sapphires.