Ruby is one of four precious gems. The world three other precious gems are sapphires, emeralds and diamonds.
All sapphires and their sister gem ruby are both made of the same material - AL² O³, the crystalline form of aluminium oxide and they are commonly called corundum. Corundum is found in every conceivable colour, even colourless. All colours are called sapphire with one exception, the red form of corundum is called ruby.
Corundum is one of the hardest natural materials (Mohs scale 9), second only to diamonds (Mohs scale 10). Rubies like sapphires can be set in jewellery, which is worn everyday and they are nearly impossible to scratch or to break. That durability ensures that ruby jewellery will be treasured for generations.

In its pure state, corundum is colourless. However, tiny amounts of trace elements create every imaginable colour. In the case of ruby, it is chromium which causes the red colour. By comparison, the blue in sapphire comes from the simultaneous presence of titanium and iron. Because titanium and iron are much more commonly found than chromium, rubies are much rarer than sapphires. For example a 10-carat sapphire in good quality is not very rare, a 10-carat ruby in good quality is nearly impossible to find.

Since ancient times gemstones including rubies and sapphires, have been heated artificially to improve their colour and dissolve tiny inclusions. This "heat-treatment" is regarded as stable and permanent.
Today the vast majority (more than 90 %) of all rubies used in the jewellery industry have been subject to heat-treatment. Heated rubies are widely accepted in the jewellery trade and must be disclosed as (E).

More recently the look of stones has been improved by filling their tiny fractures. Depending on the quantity of the fillings, these stones must be called "treated ", disclosure code (T). This new treatment must be clearly indicated in certificates.

However a very small quantity of unheated gem- quality rubies can still be found on the market. These gems always demand a substantial premium because of their rarity.