The Corundum Minerals - Sapphires and Rubies
Corundum is aluminium oxide; apart from diamond, this is the
hardest mineral known to man. In industry corundum is used as an
abrasive. Emery is corundum with small amounts of haematite
or magnetite, which explains its black colour.
Although very hard, like other gems the stones are somewhat brittle, and can be cracked by a violent blow. However they are less prone to cleavage than diamonds, which is one reason they are so useful in industry.
Synthetic stones are produced and are cheap. They are used for bearings (e.g. watches). They are not popular in jewellery, although examples do occur. In the UK, synthetic stones must be declared as such. If not described as synthetic, you can assume the stones are natural. If buying on holiday, obviously care is needed. It is wise to buy from shops with a good reputation.
They are easily cleaned with any proprietary cleaner or detergent, but do check out our advice on cleaning jewellery. If the jewellery is valuable, it's wise to obtain professional help.
Treatments before purchase
Most stones are heat-treated to enhance their clarity.
Assume this has been done, unless you are advised
otherwise. The results are permanent.
Untreated stones are available, but seek expert advice if you want to buy one, as they are much more expensive for the same clarity, and treatments can be hard to identify.
Fractures are sometimes filled. Rubies are a major problem, fractures are often filled with lead glass. This can make useless opaque stones look like good gems. But the treatment is far from permanent, and can be damaged by household chemicals, ultrasonic cleaning (as used by jewellers, and sometimes at home) as well as in other ways. The glass is much softer than ruby, so, the stone will deteriorate with time.
At first the results look attractive, but they are not the same as a real ruby. If a real ruby is too expensive, consider a synthetic, it will last much longer, and look better. They are hard for the buyer to identify, so only purchase from reputable dealers. Jewellers can easily identify them. See an example at the foot of this page
While the blue form is the most common, other colours do
occur. The red form is Ruby, however the pink variant is
called Sapphire! The best stones are a bright transparent colour,
neither very pale nor near black.
The ancients believed that simply looking upon this stone would bring good luck. Visitors to Apollo's Oracle were expected to wear a blue sapphire to ensure their questions were answered. Sapphire is the birthstone for September.
Most crystals contain inclusions or fractures which reduce the
stone's clarity so good clean stones are highly prized,
although sometimes crystalline impurities can be beautiful.
The ancients believed high quality rubies protected the wearer from danger - but the effect would be lost if these stones mixed with lower quality rubies! They are July's birthstone.
These lovely stones show a star which moves when the stone is moved. Click here for more information.