A ruby is one of the four precious gem stones (emerald, diamond and sapphire being the remainder) and is formed from the mineral corundum (aluminium oxide) with chromium present. Sapphires are also a variety of corundum with the blue colour caused by iron and titanium being present.
The name ruby comes from the Latin word ‘ruber’ meaning red. The colour ranges from pink through to a deep blood red which is caused by chromium being present in the mineral.
The cost of each stone is mainly decided by the colour, a bright deep red stone will command the highest price over rubies of a similar quality. Like diamonds, clarity plays a large role in determining the price you will pay. Stones free of inclusions are most sought after, but this may also indicate that the stone has undergone a treatment. The cut and carat weight will also be considered in determining the price.
Rubies are the third hardest naturally occurring gemstones with a rating of 9.0 on the Mohs scale. Diamonds (10) and moissanite (9.5) are the only materials with a higher hardness rating.
The colour of a ruby is caused by the presence of chromium which causes yellow/green light to be absorbed and re-emitted as a red luminescence.
Effectively every natural ruby will contain imperfections of colour impurities and rutile needles (silk) which help to distinguish them from synthetically made rubies. Nearly all rubies will be treated (usually with heat) before being cut to enhance their colour. Rubies that are certified untreated and of excellent quality will command high prices.
Corundum gems with a colour ranging from pink to deep red are generally called rubies, however a minimum level of colour saturation is required or the stone will be called a pink sapphire. This distinction is relatively new and it is not always absolutely clear where that line is drawn and has become a topic of debate amongst gemologists.
For some centuries the vast majority of the worlds rubies came from the Mogok Valley in Burma but recently very few good quality rubies have been found there. Rubies come from many deposits found around the world in Thailand, India, Pakistan, Cambodia, Vietnam and Afghanistan. Some small quantities have been found in the United States, but recently large deposits have been found in reenland as the ice shelf recedes.
As with diamonds, the system for determining the value of a ruby includes colour, cut, clarity and carat weight, in addition to their origin geographically. In valuing a ruby, the single most important factor is colour. The three components of colour are hue, saturation and tone. Hue is the term most similar to what we would refer to as colour in everyday life. Saturation is the depth of hue and tone refers to the brightness.
The primary hue of a ruby must be red and the secondary hues may include purple, violet, orange and pink. Any corundum with another primary hue will be called a sapphire.
The most sought after hue is a vivid or bright medium dark red. A secondary hue of purple helps to enhance the red making it richer and is the preferred variation. Blood red rubies from Burma were set in pure gold as it is a very yellow saturated colour. This predominance of yellow helps to neutralise the blue/purple tones and make the ruby appear to be a pure red colour.
It is common practice to treat rubies to improve their quality and the majority of rubies on the market will have been treated in some way. These treatments are used to improve or alter colour, increase transparency by removing rutiles or repair fractures in the gem.
Heat treatment is the most common and is used on uncut stones to help remove blue patches, purple tinges and silk. Temperatures approaching 1800 degrees Celsius are required although other methods using lower temperatures of around 1300 degrees and a time of around 20 to 30 minutes can also improve the colour without completely removing the silk.
Fractures in the gems can also be filled with lead glass to dramatically improve the transparency of the stones and enable otherwise unsuitable gems to be used in jewellery. This is a four step process involving polishing, cleaning with hydrogen fluoride and heating. The fourth step involves further heat after the ruby has been dipped in oils, covered with a powder containing lead glass (and optionally metal oxides to enhance the colour) which heals the fractures. The fourth part in the process can be repeated a number of times.
Synthetic rubies were first made in 1837. Potash Alum was fused with a chromium pigment at high temperatures resulting in the first synthetic rubies. Advances saw a flame fusion process in 1903 whereby commercial quantities became viable. In 1907 a 30 furnace facility was producing around 1000 kilograms of synthetic rubies annually. Other methods devised later include hydrothermal, flux and pulling processes. Flame fusion is the most economically viable method and is still used widely today.
Synthetic rubies are flawless to the naked eye, but viewing them with magnification may reveal gas bubbles, curves and striae. The less the number of imperfections, the higher the value will be. Rubies with no imperfections are extremely rare and most likely to be synthetic.
Imitation rubies are usually garnets, coloured glass or spinels that are falsely passed off to be genuine rubies. A ‘balas’ ruby is the sometimes misleading term for a red spinel, a ‘rubellite’ is another name for red tourmaline.