The Corundum Group

Ruby and Blue Sapphire are both part of the corundum group

Rubies and sapphires are part of the corundum group. On the Mohs scale of hardness they are 9. The jump in hardness between corundum and diamond is much more than it seems from the scale. A diamond is 140 times as hard as corundum, but corundum is very hard wearing… seven times as hard as topaz. Corundum can be cut either en cabouchon or faceted.

cushion cut Ruby

The red corundum is called ruby, from the Greek name for red – rubeus. There is no demarcation between ruby and sapphire – they are the same chemical composition but the colour pigment influencing ruby is chrome. The most sought-after colour is “pigeon blood” red – pure red with a slight blue hue. The names “Burmese’ or ‘Siam Ruby” refer to colour and not to origin. Large rubies are very rare. Good colour rubies are one of the most expensive gems. Large rubies are rare and therefore more expensive than comparable size diamonds. Rubies are hard but brittle, so care must be taken when setting and cutting.

Large rubies are very rare. Good colour rubies are one of the most expensive gems. Large rubies are rare and therefore more expensive than comparable size diamonds. Rubies are hard but brittle, so care must be taken when setting and cutting.

Imitation rubies have been on the market since the beginning of the 1900s. They have the same chemical composition but are man-made or manufactured, their most common name being “reco-rubies” meaning reconstituted rubies. The easiest way to detect a genuine from a synthetic gem, is from the natural inclusions found in a natural ruby when the crystal grows. Many people see inclusions in stones and think that this de-values the stone. On the contrary, this would be a sure way to tell that they are genuine as opposed to synthetic. Synthetic gems appear “too good to be true” with no flaws.

blue sapphire gemstone
Any stone referred to as a sapphire is blue, all the other colours are qualified by description i.e. yellow sapphire, green sapphire, pink sapphire, violet sapphire, and red sapphires. The colouring pigment in blue sapphire is iron and titanium; in violet stones, vanadium. A small iron content results in yellow and green tones, whereas chrome produces pink. The most highly sort after colour is “cornflower blue” which used to be called “Ceylon sapphire”. Like Burma-rubies, this refers to the specific colour as opposed to where they are mined. Sapphires are more common than rubies, and as with rubies you can get “cat’s eye” sapphires. Most common deposits are found in Burma, Sri Lanka, Thailand and India. Large stones are rare, and synthetic sapphires have also been on the market since the beginning of 1900. The same methods are used in recognising a synthetic from a natural, by the inclusions and colour banding which are very specific to corundum.