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Crystal system:    Trigonal (hexagonal scalenohedral)        Hardness:    9 (defining mineral)

Density:    4                        Streak:        white

Cleavage:    None                    Composition:    Oxide mineral    Al2O3

Being the Christmas season I was requested that the MOM must be related or associated with the season – possibly a mineral of which the name resembles or relates to Christmas?   No luck there…. I could not find Christmasnite, Rudolphnite or St. Nickelsite in any book. Then I started looking at colours or maybe something red, white or green which are typical colours associated with Christmas. So I stumbled onto something red, something precious, something pretty, something that was for many centuries regarded as the birth stone for December, and something most of us would not mind finding in our stocking or under the tree on Christmas day …. a ruby.

A ruby is a pink to blood-red coloured gemstone which is a variety of the mineral corundum (aluminium oxide). The red colour is caused by the presence of minor quantities of the element chromium which replaces aluminium in the crystal structure. The name comes from “ruber”, Latin for red and as a matter of interest….or to add confusion, the name “corundum” is derived from the Tamil word “kuruntam” meaning “ruby”. Other varieties of gem-quality corundum are called sapphires. The ruby is considered one of the four precious stones, together with sapphire, emerald, and diamond.

Corundum occurs mainly in metamorphic rocks and as large crystals in pegmatites. The largest documented single crystal of corundum measures 65 cm x 40 cm x 40 cm. Rubies normally occur as well-formed crystals showing good form. The crystals can sometimes fluoresce under long wave UV light (take note Jo).

International ruby localities include, Afghanistan, India, Burma, Vietnam, Russia, Tanzania (ruby embedded in green zoisite).

Considerable controversy seems to surround the question of the world’s largest ruby. One of the most famous is the Rajaratna Ruby which is 2 475 carats and was unveiled to the world at the end of 1986. Its owner, Mr G. Vidyaraj of Bangalore, India, inherited the stone from his ancestors, the kings of the Empire of Vijayanagar. Since then, the 125 West Ruby, which is certified by the Gemological Institute of America and weighs 18 696 carats, has claimed the prize, as has another stone from Myanmar reported to be 21 450 carats.

Prices of rubies are primarily determined by colour. The brightest and most valuable “red” is called pigeon blood red and commands a large premium over other rubies of similar quality. In traditional gemological terms, ruby has to be blood-red and of a clear, facetable quality to be considered to be a real ruby. In the wider usage, however, any corundum with a red or reddish colour has obtained the name “ruby”, and this name is usually applied in this way by mineral collectors.

Rubies have a hardness of 9 on the Mohs’s scale of mineral hardness. It can, therefore, scratch almost every other mineral. Among the natural gems only diamond is harder with a Mohs’s hardness of 10.  Ruby corundum is, therefore, commonly used as an abrasive, on everything from sandpaper to large machines used in machining metals, plastics and wood. 

The largest corundum crystal in the world, a grey 59 cm long stone weighing 151 kg, which is displayed in the Transvaal Museum in Pretoria, was found in the Limpopo Province. I am, however, not sure whether this refers to the same corundum specimen referred to in paragraph three above, since the dimensions differ slightly.

Corundum was mined from the early 1900s in a broad area from Polokwane to Musina to Leydsdorp. True gem quality ruby does not occur in South Africa, although some translucent ruby-red corundum occurs in grey-white host rock, south of Aggeneys, in the Northern Cape.

Corundum is not widespread in Namibia and some grey-brown to pale pink corundum is found in the Karasburg district. It is also found at Kyanite Kop, a prominent hill composed of corundum, kyanite and diaspore in the Windhoek district. Other districts where it is found include Omaruru, Damaraland, Karibib and Warmbad.

Zimbabwe was once, after Russia, the world’s largest producer of corundum. Some small faceting quality stones have also been found.

In 1837 Marc Antoine Gaudin made the first synthetic rubies by fusing alumina at a high temperature with small amounts of chromium as pigment. Later the “Verneuil process” (after Auguste Verneuil) was developed that allows the production of flawless single-crystal sapphires, rubies and other corundum gems of much larger sizes than normally found in nature.  JDJ

All photos and specimens supplied by Johann de Jongh
References
Bruce Cairncross,Field Guide to Rocks & Minerals of Southern Africa (2004).
http://en.wikipedia.org
http://webmineral.com
www.mindat.org