Martin Rees, Jeweller and Pawnbroker

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.

Star Rubies

These lovely stones show a star which moves when the stone is moved. Click here for more information.

Ruby with large crystal inclusion
Above 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).

Ruby with fractures filled with lead-glass, showing typical blue flashes

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An old sapphire seen through a microscope
Above, This sapphire is about 90 years old.  You can see signs of wear, also an attractive colour band, common in natural sapphire. Click here for larger image (135KB).

Cut sapphire with zircon crystal
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. Click here for larger image (323KB).

Sapphire with rutile
Above A silky veil of rutile crystals in a very clear sapphire.
This picture was a challenge.  Although the stone looks dark in fact it's almost a white sapphire, with just a touch of blue.  The veil was almost invisible, and required very careful lighting.  I tried moving the light to reduce the reflections and the veil almost vanished!  Click here for larger image (109KB).

Square cut ruby
Above a square-cut ruby.  Note the fractures, one from top left towards bottom right, the other crossing it.  The stone is well cut, and the fractures go straight down into it, minimising their impact on its clarity.  also you can see two colours, a blue-red, and a more yellow red.  This is a characteristic of ruby, caused by the way the crystal refracts light.

Synthetic ruby showing characteristic bands
Above the bands across this stone tell us it is synthetic.  The pattern of growth causes these features.  The facets are out of focus because we are actually looking beyond them, inside the stone.  While these bands easily identify synthetic stones, their absence is not proof that a stone is natural.  Careful inspection is required to be sure of a gem's origin.  Click here for larger image (161KB).