Ruby

A Ruby is a pink to blood-red gemstone, a variety of the mineral corundum (aluminium oxide). The common red color is caused mainly by the presence of the element chromium. Its name comes from ruber, Latin for red. Other varieties of gem-quality corundum are called sapphires. The ruby is considered one of the four precious stones, together with the sapphire, the emerald, and the diamond.

Prices of rubies are primarily determined by color. The brightest and best "red" called pigeon blood-red, commands a huge premium over other rubies of similar quality. After color follows clarity: similar to diamonds, a clear stone will command a premium, but a ruby without any needle-like rutile inclusions will indicate that the stone has been treated one way or another. Cut and carat (size) also determine the price.

Physical properties

Rubies have a hardness of 9.0 on the Mohs scale of mineral hardness. Among the natural gems

Ruby
Ruby crystal before faceting, length 0.8 inches (2 cm)
General
Category	Mineral variety
Chemical formula	aluminium oxide with chromium, Al2O3::Cr
Identification
Color	Red, may be brownish or purplish
Crystal habit	Varies with locality. Terminated tabular hexagonal prisms.
Crystal system	Trigonal
Cleavage	No true cleavage
Fracture	Uneven or conchoidal
Mohs Scale hardness	9.0
Luster	Vitreous
Refractive index	~1.762-1.770
Pleochroism	Orangey red, purplish red
Ultraviolet fluorescence	red under longwave
Streak	white
Specific gravity	4.0
Melting point	2050°C
Solubility	none
Diaphaneity	transparent

only moissanite and diamond are harder, with diamond having a Mohs hardness of 10.0 and moissonite falling somewhere in between corundum (ruby) and diamond in hardness.

All natural rubies have imperfections in them, including color impurities and inclusions of rutile needles known as "silk". Gemologists use these needle inclusions found in natural rubies to distinguish them from synthetics, simulants, or substitutes. Usually the rough stone is heated before cutting. Almost all rubies today are treated in some form, with heat treatment being the most common practice. However, rubies that are completely untreated but still of excellent quality command a large premium.

Some rubies show a 3-point or 6-point asterism or star. These rubies are cut into cabochons to display the effect properly. Asterisms are best visible with a single-light source, and move across the stone as the light moves or the stone is rotated. Such effects occur when light is reflected off the silk (the structurally oriented rutile needle. inclusions) in a certain way. This is one example where inclusions increase the value of a gemstone. Rubies can furthermore show color changes — though this occurs very rarely — and chatoyancy.