The Corundum Minerals Sapphires and Rubies Corundum Minerals

A natural,uncut ruby in matrix.  Most pictures on this site are my own work, I had to buy this one, I don't own an uncut ruby.
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Above a faceted ruby with a large crystal inclusion.  The crystal fractured the stone, and the fracture 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, so the crystal looks distorted, caused by different refraction through 2 facets.  Click here for larger image.

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.  You can see two colours, a blue-red, and a more yellow red (bottom left and right edge).  This is a characteristic of ruby, caused by the way the crystal refracts light.

Above the bands across this stone tell us it is lab-grown.  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 lab-grown stones, their absence is not proof that a stone is natural. Click here for larger image.

In brief

After diamonds, these are the hardest gems

They're easy to clean

Most gems are heat treated to improve clarity

Full info about 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, that's why it's black.
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 this than diamonds, which is one reason they are so useful in industry.
Lab grown stones are cheap.  They are used for bearings (e.g. mechanical watches).  In the UK, lab-grown stones must be declared as such.  If not described as lab-grown, assume the stones are natural.  If buying on holiday, care is needed.  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 best to get professional help.

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Above, This sapphire is early 20th Century.  You can see signs of wear, also an attractive colour band, common in natural sapphire. Click here for larger image (opens in a new tab).

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.  There are bands of rutile needle crystals below the fracture halo. Click here for larger image (opens in a new tab).

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 very pale blue.  The veil was almost invisible, and required careful lighting.  I tried moving the light to reduce the reflections and the veil vanished!  Click here for larger image.

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 not permanent, and can be damaged by household chemicals, ultrasonic cleaning and 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 clear ruby.  If a good clear gem is too expensive, consider lab-grown, it will last much longer.  Treated stones are hard for the buyer to identify, so only purchase from reputable dealers.  Jewellers can easily identify them.

Below 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.

Sapphire

While the blue form is the most common, other colours do occur.  The red form is Ruby, however the pink is called Sapphire!  The best stones are a bright transparent colour, neither very pale or 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 gems contain inclusions or fractures which reduce the stone's clarity, good clean stones are highly prized, 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.

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