Alongside colour and lustre, sheen is an optical property that engenders beauty in a gem.
This property is often described as giving certain gems a ‘mystical’ appearance.
In the ancient world, a gem that displayed sheen belonged to the gods and was believed to impart magical powers.
Sheen – which is caused by the reflection of light off a gem’s internal inclusions – is seen in a variety of forms within the gem world, and each form has a different gemmological name.
For example, iridescence is the form of sheen that makes moonstone and labradorite so attractive to our eyes.
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| Above: Asterism - From Ancient Greek aster, meaning ‘star’ |
Two unusual and interesting forms of sheen are asterism and chatoyancy.
Gems that display asterism and chatoyancy contain inclusions that reflect light in a particular way.
How these inclusions are arranged will determine if a gem displays asterism or chatoyancy.
If we look into a gem and see a star shape, this form of sheen is called asterism.
The term comes from the Ancient Greek word aster, meaning ‘star’.
Gems that display asterism are highly sought by collectors.
The gem type, the clarity of the star and number of star points – four, six or 12 – are factors that can influence its value.
To highlight the star and show its optimal effect, the cutter will fashion the gem as a cabochon.
Sapphire and ruby, diopside, garnet and rose quartz can display asterism.
When asterism is present, the gem is named by using ‘star’ as a prefix, followed by the gem name, for example, star sapphire or star ruby.
Asterism occurs when light is reflected off inclusions of fine crystals or fibres that are orientated at specific angles to each other.
In a star sapphire, fine needles of hematite or rutile intersect at 60 degrees.
In a star diopside, the fibres intersect at 73 degrees.
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| Above: Chatoyancy - From French oeil de chat, meaning ‘cat’s eye’ |
Light moves across the surface as the gem’s orientation changes. In turn, the strength and brightness of the star changes.
Chatoyancy refers to sheen created by light reflecting off parallel channels, crystals or fibres included within the gem.
The term chatoyancy comes from the French term oeil de chat, meaning ‘cat’s eye’.
A chatoyant gem displays a single bright band of reflected light across its surface.
Just as a cat’s eye, seen at night, shows a single line that changes as it moves, the band on a chatoyant gem also alters its position as the gem is moved under a light source.
To highlight the cat’s eye effect, the cutter will use a cabochon cut.
Many gems can display chatoyancy, including tiger’s eye – a member of the quartz gem family – chrysoberyl, emerald and tourmaline.
Chatoyant gems are named by using the gem’s name first, then the term ‘cat’s eye’ as a suffix, for example, tourmaline cat’s eye or chrysoberyl cat’s eye.
Sheen is an important optical property that contributes to the beauty, value and desirability of many gemstones.
Asterism and chatoyancy are two forms of sheen that make the gem world so exciting and enticing.