The Science Behind The Gems
Sparkly. Pretty. Glittery. Fabulous. Delicate. Exquisite. Rare. Precious. All adjectives that describe the gemstones we all see and love in a jewellers window.
BUT, there is so much more to them than outward beauty. What makes them the way they are and how can we be sure what they are?
All gemstones are either crystalline or non-crystalline. The majority are crystalline and this means that the gem has a regular internal atomic structure and , in the crystal’s fully developed form this gives rise to geometrically arranged external faces. The crystalline gems each belong to one of the seven crystal systems: Cubic, Tetragonal, Hexagonal, Trigonal, Orthorhombic, Monoclinic and Triclinic.
Diamonds, Garnets, Fluorite and Spinel are examples of Cubic gemstones. Zircon is from the Tetragonal system. Emerald, Apatite and Synthetic Moissanite (a diamond simulant) are Hexagonal gems. Corundum (Sapphires and Rubies), and Quartz are Trigonal. Chrysoberyl, Iolite, Marcasite, Peridot and Topaz are some of the more well known Orthorhombic gems. Jadeite, Nephrite, Malachite and Diopside are a selection of the Monoclinic gems and finally Labradorite, Rhodonite and Turquoise are Triclinic gems.
In addition to the crystalline gems there are those that are non-crystalline or amorphous (without form) these are gems that include Opal, Jet, Moldavite and Obsidian (two natural glasses), Amber, Ivory and tortoise-shell.
Some doubly refracting gemstones not only split the light into two rays but each ray can be viewed as a different colour or hue. Tourmaline is a dichroic material and when you look at a Green Tourmaline from one direction it will look green and from another brown. The Dichroscope contains a calcite rhomb which is a very highly doubly refractive material and allows the two light rays to be seen side by side.
A Polariscope consists of two polarising discs at 90 degrees to each other. When a singly refracting gem is placed between the two polars and is rotated, the stone will remain dark as it is turned. With a doubly refractive gem there will be some parts of the rotation where the gem is dark and some parts where it is light, this is due to the two directions of the light rays within the gem. There are advanced uses for this instrument which help to identify which particular doubly refractive crystal system the gem could belong to but that would be a little too in depth for this article.
I am sure you are aware of the way light splits in to it's component colours (colours of the rainbow) when it passes through a glass prism. A Spectroscope is a small hand held instrument that allows gemmologists to examine the individual spectrums exhibited by some gemstones. When you look through the spectroscop at a light you will see the rainbow of colours set out side by side. Usually in the UK we look at a spectrum with the red part on the left and the blue on the right hand side but it really does not matter which way round you look at it as the important parts will be in the same place whichever way round you view it. You are not looking at what you 'can' see, rather what you 'can't see'. Certain parts of the spectrum will be absorbed by the gemstones depending on which chemical element produces the colour of the gem. This absorption can be a fine line, heavy line, a whole area blocked out or a combination of all of these. In some gemstones fluorescence is given off (as with Ruby) and you can then see a very bright line (an emission line). A spectrum can be diagnostic of a particular gem but not all gemstones show a spectrum.
All gems whether crystalline or amorphous have a 'refractive index'. As a light ray passes through a gem it is bent and a refractometer measures the amount by which the ray is bent. In Cubic gems and non-crystalline gems the light ray simply bends and these materials are described as 'Singly Refractive', but in all the other crystal systems the light ray not only bends but it is also split into two rays that vibrate in two directions at 90 degrees to each other. These gems are 'Doubly Refractive'. An instrument called a refractometer can measure how much the light bends and, with doubly refracting gems it can measure the difference between the two rays. These readings can help to differentiate between gemstones as, for example, a blue sapphire and a blue topaz will both show two readings but they will be different for each gemstone and the difference between the two readings will also be different for each.
Inclusions in gemstones take many forms, from gas bubbles to liquid and solid crystals. The inclusions seen have not just arrived, they were there at the same time that the gemstone formed and the gem crystal grew around them. Generally speaking, it is preferable to have a gemstone that is as free as possible from inclusions, however, it can be the deciding feature that determines whether a gemstone is natural or a man made laboratory grown synthetic crystal. Many natural sapphires when examined under a jewellers 10x magnification loupe will show straight lines of colour banding, this is where the crystal growth has been interrupted and re-started. In man made synthetic sapphires the bands still exist but they are curved line rather than straight due to the way that the molten material forms in a 'boule' - a round ended cylindrical shape. Some emeralds exhibit particular spiky 'three phase' inclusions (ones that show a liquid filled cavity with a bubble and solid in it.) and the presence of these inclusions not only identifies them as natural but also identifies the mine they were found in. Many people see 'black marks' inside their diamonds and say it is full of carbon - in reality the inclusions they are seeing may be a variety of substances. Peridot often have a typical inclusion that is reminiscent of a lily pad. The inclusions in gemstones are like their fingerprints as no two will be exactly the same.
The first step towards identifying a gemstone is looking at it. Jewellers use a 10x magnification loupe for most identification and examinations. The lenses in these loupes should be aplanatic and achromatic meaning that they sharpen the image towards the edges of the field of vision and straighten the light rays. A loupe should be held close to the eye and then the item under examination brought to it until it is in focus - a distance of about 5cm from the lens. In addition to the hand lens (loupe) jewellers also use microscopes to further examine the interior of the gemstones.