Why are Rubies Red?

Pigeon Blood Red Ruby Cabochon by The Ruby King

The red color of the rubies is attributed to the presence of Chromium ions as an additional element in Corundum.

Ruby is α-alumina (the most stable form of aluminum oxide) in which a small fraction of the aluminum3+ ions are replaced by chromium3+ ions. Each Cr3+ is surrounded octahedrally by six O2- ions. This crystallographic arrangement strongly affects each Cr3+, resulting in light absorption in the yellow-green region of the spectrum and thus in the red color of the gem. When yellow-green light is absorbed by Cr3+ , it is re-emitted as red luminescence. This red emission adds to the red color perceived by the subtraction of green and violet light from white light, and adds luster to the gem's appearance. 

The color of Ruby ranges from Pinkish Red to Blood Red & even Dark Red The best red (popularly known as Pigeon Blood Red), and thus most valuable, rubies are usually from Burma. Dark and Bluish red rubies come principally from India. The most preferred color is a deep blood red with a slightly Pinkish hue.

Various shades of Ruby

 

Asterism in Rubies

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, stimulants, or substitutes.

Corundum owes its property of asterism to these aligned microscopic inclusions, which create the appearance of a "star" in a round cabachon-cut stone. Some rubies show a 3-point or 6-point asterism. Asterisms are best visible with a single-light source, and move across the stone as the light moves or the stone is rotated. Rubies can show color change — though this occurs very rarely — as well as chatoyancy or the "cat's eye" effect.

Rosser Reeves Star Ruby

 

Inclusions in Ruby Crystal

An inclusion is any irregularity observable in a gem – such as a solid mineral crystal or a fluid filling a cavity, or it may be an unfilled cavity, a fracture, or a growth pattern that produces some optical effect.

The various inclusions in Ruby are-

Pre-existing inclusions (protogenetic):

These are the inclusions that are formed before the host. They are strictly of a solid nature (pre-existing liquids and gases don't count). Corundums which formed in metamorphic environments, such as Burmese rubies, are often rich in solid inclusions.

Light yellow apatite crystals in a corundum from Umba Valley, Tanzania; 25x. Photo courtesy: GRS Gem Lab
 

Contemporary inclusions (syngenetic):

Inclusions that have formed at the same time as the host. Crystals and/or glasses that form, and are trapped, at the same time as the host. These consist of cavities formed while the host itself was growing. The examples may be calcite and dolomite in ruby from metamorphic environments (such as Mogok, Burma).

Terraced basal pinacoid faces on Burmese ruby crystals. Photo courtesy: Ruby-Sapphire

 

Secondary inclusions (epigenetic):

Inclusions that have formed immediately, or even millions of years, after the host stopped growing. The secondary inclusions may be present in form of  exsolved crystals, healed fractures or Secondary Twinning in crystals.

Two systems of crossing twinning lamella present in an unheated "Winza"-Tanzanian ruby. Photo courtesy: GRS Gem Lab

Have more questions regarding the crystal structure of Rubies, ask them in the comments below

Come back tomorrow to know “What does hardness of 9 at Mohs scale mean?”

Best regards
Sambhav Karnawat
K.K. Exports
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