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.
Cleaning
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
Sapphire
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.
Ruby
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
Below a faceted ruby with a large crystal inclusion. This crystal fractured the stone, which fracture then filled with micro-crystals, which are just visible as a fringe around the crystal. Unfortunately the faceting prevented us from protographing the stone from the best angle, and also you can see what looks like distortion of the crystal, which is caused by different refraction through 2 facets. Click here for larger image (90KB).
Below an example of a treated stone. This stone was seriously fractured, almost opaque, and unsuitable for jewellery. Lead-glass was forced into the fractures, producing a surprisingly clear stone.
How do we know it was treated? First you can see a lot of faint flaws. But the real giveaway are the blue flashes, which come from the lead-glass.
Click here for
larger image (164KB).
Above a zircon crystal embedded in the
sapphire has caused the stone to fracture. The fracture is as thin as an
oil film, hence the way it refracts the light. Although it looks so
spectacular, the light has to be at exactly the right angle; normally the
fracture is almost invisible. Note also the bands of rutile needle crystals
below the fracture halo. 
