Martin Rees, Jeweller and Pawnbroker

Why are gems beautiful?

The charm of crystals

All solids tend to form crystals - even metals when examined under the microscope are crystalline. To prevent crystals forming the molten material must be cooled quickly - as is glass. Almost all gems are crystalline, except opal, and the organic gemstones (pearls and amber).
In crystals the atoms are arranged in order, and as a result most single crystals are transparent to light. The light will be refracted internally, often split into 2 rays, and may show the same rainbow effects as if passed through a prism. While some minerals are always coloured, quite often it is impurities within the crystal which cause the different colours - somehow impurity seems the wrong word to describe what causes the beautiful colour of gemstones! These impurities are traces of other elements, which don't form part of the basic crystal structure.
Some gemstones consist of many small crystals, which do block the light and appear opaque.  Agate is a popular example.
To early man, these crystals must have seemed miraculous - how could something with a geometric form and such transparency be natural? Small wonder that they were believed to be gifts from the gods, and deemed to have mysterious powers. I have only listed a few of the many traditions linked to gemstones, but I hope you find it interesting, and it will give you some idea of how man has always respected these stones.

Understand how crystals crack - causing fracture planes

Everybody knows gemstones are hard.  But the alignment of the atoms in the crystal causes some very curious effects.  Gemstones are harder in one plane than in others.  While not a crystal, consider the way water forms a skin, so that insects can stand on it.  What happens is that electrons at the surface are unable to form the usual bonds with other atoms, and this causes surface tension.
Mica is an extreme example.  The mineral is quite tough, yet comes in extremely thin slices which can be easily split apart.  In gemstones the plane of minimum strength is called the cleavage plane.  Diamonds can be easily split along that plane.
This tendency to split along planes helps us to understand the fractures which occur in gemstones.
Many gemstones, while hard are also brittle.  A hard knock, especially along the cleavage plane, can damage a stone.  So always take good care of your jewellery.

Through the microscope

Gems often look perfect to the naked eye.  But magnify them and we see hidden details.  They are flaws in the perfect crystal structure.  But they are often beautiful, and help identify gems.

Sapphire cracked by zircon crystal
Above - inside a sapphire
Note the small dark spot at the centre of the fracture halo, it's a crystal of zircon.  It set up stresses in the sapphire which caused it to crack.  The crack is so fine that it causes interference patterns in the light it reflects.  Below it, bands of rutile crystals cause colour-zoning which is quite common in sapphires.  Together they prove the stone is natural, and has not been heat-treated.   Click here for larger image (261KB).

Synthetic spinel
Flaws in a man-made stone
We thought we were checking a CZ, until we saw this line of extended bubbles.  They proved the stone to be synthetic spinel.  Click here for larger image (187KB).