The Ruby (Corundum) is the rarest of precious stones used in jewellery. The name of this gem comes from Latin word Rubeus meaning « red ».
It’s one of the most expensive precious stones : bigger Rubies are rarer than diamonds of the same size.
Rubies symbolise splendour and joy.
History, deposits and Ruby mining
Very late onwards, at the end of the 18th century, Rubies started being incorporated like Sapphires into the group of Corundum.
Before that, the Red Spinel and the Garnet were often considered Rubies. These 3 stones were red and used to be called “carbuncles”.
The mother-stone of the Ruby is composed mainly of dolomitised marble born from the metamorphism of granite with limestone. The proportion of the Ruby in primary deposits of this kind is too small to be commercialised.
Alluvial deposits allow for a much greater production. The specific high weight of the Ruby allows its collection in concentrated residue, obtained by washing the gravel of fluvial sands. Rubies are then sorted by hand.
The biggest mines are in Myanmar (Burma), Thailand (Siam), Sri Lanka (Ceylon), Tanzania, Madagascar and Vietnam.
Northern Burma, around Mogok, has the most abundant production. The Rubies-full layer is located several metres underground. It’s exploited through the use of shafts and galleries. Only a very small quantity of stones is intended for jewellery. Rubies extracted from these deposits are usually known as Burmese Rubies.
The colour of Rubies from Thailand is bright red with a slight brown overshadow. Rubies can be found in clayey gravel in the region of Chantabun at the south-east of Bangkok. The exploitation galleries can as big as 8 metres in depth.
Rubies from Thailand are often called Siam Rubies.
In Sri Lanka, Ruby deposits are located in the region of Ratnapura at the Southwest of the island. They’re extracted from rocks and sands from the depths of rivers.
For the most part, Rubies that come from these alluvials, called « illam » in the country’s language, have a colour that varies from light red to raspberry red. They are named Ceylon Rubies.
Since the fifties, Tanzania has been producing big Rubies occasionally, but they’re of lesser quality. Other deposits have recently been found in the north-western part of the country. These are purple with a red-brown nuance.
The biggest Ruby ever found weighed 400 carats. It was discovered in Burma and cut into three gems.
Here are some of the most famous stones:
- The Edward Ruby, 167 carats, housed in London’s British Museum.
- The DeLong Star Ruby, 100 carats, housed in the American Museum of Natural History in New-York, USA.
- The Peace Ruby, 43 carats, called this way for having been found after the First World War in 1919.
Colour and properties of a Ruby
The red of the Rubies and its various tones are due to chrome ions which, in small proportions, substitute the aluminium ones in the crystalline structure (composed of aluminium oxide and chrome). When the colour leans towards brown or purple, the Rubies have iron ions.
The red is not always the same within the same deposit.
The colour alone is not enough to determine the origin of the stone for certain. Consequently, names like Burmese Ruby and Siam Ruby can lead to confusion and serve the purpose of distinguishing the quality of the stone rather than its colour.
However, the natural colour of Rubies is extraordinary : there’s more orange in Eastern Rubies (Ceylon and Burma) while they are a bit garnet-coloured from Siam.
The most prized colour is bright red with a slight blue touch known as Pigeon‘s Blood. Once cut, the brilliance of the Ruby can equal that of the diamond.
Although a Ruby is 140 times softer than a diamond, it scores 9 on MOHS’ 10-degree scale, right after the diamond.
Streaks are frequent but do not alter the quality of the stone. To the contrary, they guarantee its authenticity as a real Ruby as opposed to a synthetic one.
The kinds of the inclusions that can be found (minerals, channels or liquid streaks) show many indications regarding the original location of the stone since each deposit has its own types of inclusions.
Rutile inclusions, known as silk, give the stone a sweet glint : it can be enhanced when the Ruby has a Cabochon Cut. These inclusions produce an asterisked effect. This phenomenon is very prized and makes a 6-pointed star appear which seem to be sliding on the surface of the stone when it is moved: the Ruby is called Star Cabochon Ruby.
Most Rubies are cut in their country of origin. This cut is not always satisfactory since cutters aim at keeping the stone at its maximum weight.
Consequently, many stones have to be cut again to get optimum results in jewellery. The Step Cut and Fancy Cut (pear, cushion, heart and marquise) maintain all of the stone’s transparency and brilliance. Stones with less transparency are often cut in Cabochon.