Crystals Rocks Minerals to Tempt and Tantalise You

What is a Gemstone?

colourful faceted gemstones
Contents

1. Gemstone Meaning and Powers
2. The Colour of a Gemstone
3. Magical Properties of Gemstones
4. Hardness Toughness Stability
5. Faceted Stones and Cabochons
6. Article Pictures

Gemstone Meaning and Powers

Our fascination with gemstones dates back thousands of years.  Today our bond with these natural curiosities is stronger than ever.  Common to every age and culture we're captivated by their colours, the way they react to light and the transformation that takes place once they've been cut and polished.

For many people gemstones are more than just objects of beauty.  They're believed to have a deep inner meaning, mystical powers and the ability to influence mental or physical behaviour.  They're mentioned in texts of major religions and have long been used in rituals and ceremonies.

In Imperial China some emperors, royalty and the wealthiest aristocrats were buried in suits made from jade.  It was believed the stone would bring immortality.

In Ancient Egypt gemstones were used as part of the complex burial rituals of the pharaohs.  They were said to help ease the soul into the afterlife.

In many ancient cultures it was common for people to be buried with their most prized possessions.  These items which often included gemstones have been invaluable to historians and archaeologists. 

For thousands of years the finest gemstones have been sought after by those for whom money is no object.  Jewellery and accessories featuring diamonds, emeralds, rubies and sapphires can be found in collections around the world.  Many feature gemstones considered to be priceless because of their size, rarity or beauty.

The Green Vault in Dresden, the Smithsonian in Washington D.C, The Armoury Palace in Moscow and the Tower of London home to the British Crown Jewels hold some of the most beautiful gemstones the world has ever seen.

Several factors are taken into account when determining the suitability of a crystal rock or mineral to be classed as a gemstone.  The three most important characteristics are beauty, durability and rarity but without beauty the other two mean very little.

The feature that causes a gemstone to be beautiful can vary widely.  The same characteristic can have the opposite effect when present in a different stone.

Rubies sapphires and emeralds are prized for their magnificent colour yet the finest diamond is colourless.  Gem grade aquamarine and topaz command exceptional prices when free from inclusions yet some types of quartz increase in value because of inclusions.

black rutile inclusions in transparent quartz

The Colour of a Gemstone

The colour of a gemstone will often be its most important feature.  It can also be the least reliable for accurate identification.  Identifying a gemstone based on colour alone has led to countless mistakes being made.

A fine grade gemstone must exhibit good depth of colour without being too pale or too dark.  The colour should be uniform throughout which can sometimes be an issue.  In amethyst the shade of colour often varies within the same crystal.  This characteristic known as colour zoning can be subtle or clearly visible.

Banded amethyst (also known as chevron amethyst) is the most extreme example of colour zoning.

seven chevron amethyst tumbled stones

The colour of a gemstone is caused by the absorption or refraction of light.  This refers to the way the surface interacts with light.

White light which is colourless light an example being daylight is made up of red, orange, yellow, green, blue, indigo and violet.  When light hits a surface some of it is absorbed whilst the rest is reflected.  The light that's reflected is the colour we see.

The different colours of white light can be seen through a prism.  A prism is a three dimensional translucent object with flat sides.  At least two sides have the same size and shape.  Examples include a triangle, cube, hexagon and octogon.

The prism works because the different colours in white light slow down and travel at different speeds as they pass through an object.  When they come out the other side they speed up again.

When light changes speed it changes direction or in other words it bends.  The change in direction or 'bending of light' as it passes from one medium to another is known as refraction.

With different colours being bent by slightly different amounts they come out the other side separately instead of combined.  Red bends the least whilst violet bends the most.

For refraction to take place the light must hit the object at an angle as opposed to head-on. 

Light moves more slowly when passing into a denser substance.  When travelling from air into water, through glass or a translucent gemstone it bends or refracts.

An example of refraction is a pencil in a glass of water.  At the point where the pencil makes contact with the water it appears to be misaligned.  This is caused by refraction of light. The light only bends at the point where the pencil meets the water.  It then continues in a straight line.

Another example of refraction of light is a raindrop.  Raindrops create rainbows because each one acts like a prism.  The rounded shape of the rainbow is caused because the prism which is the raindrop is also round.

pencil in a glass of water demonstrating refraction of light

The largest group of gemstones are those which are mostly colourless in their purest form.  These stones are often translucent.

The characteristic that causes colour to change in a gemstone may always be the same or can differ depending on impurities present.

The tiniest alteration in the stone's crystal structure can have a significant impact.  The green in peridot is caused by impurities of iron.  The colour of rhodochrosite comes from impurities of manganese.  In its purest form a quartz crystal is colourless but with the presence of iron (and exposure to heat) it becomes amethyst.

Corundum turns red with impurities of chromium.  Replace the chromium with iron and it turns yellow.  With titanium and vanadium corundum turns blue.

In jasper impurities of hematite produce a deep shade of red.  The green of malachite comes from copper.  The varying shades of blue in turquoise come from copper and aluminium.  When iron replaces aluminium the colour of turquoise becomes more green.  With the presence of zinc hues of yellow can be seen.

In its purest form beryl is colourless.  Various impurities introduce shades of red, green, yellow and blue.  Green beryl is emerald, blue is aquamarine.

Other factors that affect colour change in a gemstone include heat, the orientation of the crystals and the presence of a structural imperfection.  That could be some kind of damage or an anomaly that may have occurred during the stone's formation. 

Without light there would be no colour.  The human eye only sees colour because of the way light interacts with an object.  Our eyes and brain then work together to enable us to perceive the colours we see.

Objects only appear to be colourful when illuminated by light.  As the light fades so does the colour.  Our eyes then only see black and grey.

Magical Properties of Gemstones

Many gemstones exhibit visual effects that are often described as optical phenomena.  These characteristics alone can lead to a significant increase in value.

They're caused by the reflection or refraction of light and in some cases both.  Light may reflect off the surface of a stone or off its inclusions.

The word lustre is used to describe the way light reflects off the surface of a gemstone.  Although the type of lustre known as vitreous tends to be the most common, there are several others.  Polishing a gemstone will almost always improve its lustre.

Lustre should not be confused with brilliance which is the reflection of light from within a faceted gemstone.

Some of the terms used to describe optical phenomena in gemstones include play of colour, adularescence, schiller, labradorescence, aventurescence, chatoyance and asterism.

Play of colour in precious opal is caused as light bends as it squeezes through tiny gaps between silica spheres.  The result is a spectrum of colours.

The iridescence in labradorite is caused as light enters the stone and reflects off different layers.  The colours we see are the reflection of light.  Although correctly known as schiller, the term labradorescence was coined because labradorite is renowned for this optical phenomenon.

labradorite cabochons in different sizes some showing labradorescence

The word adularescence is often used to describe the same effect when present in moonstone.  As light reflects off different layers a milky white glow or distinctive blue sheen can be seen.

Aventurescence is an optical phenomenon seen in aventurine and sunstone.  Asterism produces a four or six rayed star in some cabochon-shaped gemstones.  The effect is caused as light reflects off crystal inclusions of rutile.  

Chatoyance often seen in tigers eye and chrysoberyl is caused as a parallel band of light reflects off fibrous inclusions within the stone.  As the direction of light changes the band of light moves.  This gives the impression of movement on the surface of the stone. 

The one thing all optical phenomena have in common is they're caused because of the way light interacts with the surface of the stone or its crystalline structure.  The extent to which it can be seen is dependant on the angle of light and angle from which the gemstone is being observed.

Hardness Toughness Stability

Durability is another factor that determines the suitability of a crystal rock or mineral to be classified as a gemstone.  This characteristic is comprised of three parts, hardness, toughness and stability.

Hardness and toughness are often looked upon as being one and the same but are different.

Hardness relates to scratch resistance meaning how easily one stone can be scratched by another.  Mohs scale of mineral hardness is widely used for this purpose.

Toughness relates to a stone's resistance to cracking, chipping, pressure or breaking.  With regards to a gemstone, toughness affects how easy it is to facet.

Whilst diamond may be the hardest mineral known to man it's not the toughest.  Diamond grades 10 on Mohs scale whilst nephrite jade grades 6 to 6.5.  Nephrite is tougher than diamond because of its strong interlocking network of fibrous crystals.

Pearl grades 3 on Mohs scale yet is very tough.  If dropped onto a hard surface from height it's unlikely to break.  Do the same with a diamond and it will smash to smithereens.

Topaz is another hard mineral which grades 8 out of a possible 10 on Mohs scale.  Despite its hardness it's fragile so will chip very easily.  It's also sensitive to heat and pressure both of which can cause the stone to crack.

When searching for information online regarding toughness in relation to rocks minerals or gemstones, all search results lead to Mohs scale of mineral hardness.  The only way of finding information about this characteristic is to use the correct geological term which is 'tenacity'.

Stability refers to a gemstone's resistance to damage through chemicals or a change in structure caused by 'deteriorating forces'.  Using turquoise and malachite as examples, both minerals are delicate and porous hence can easily be damaged by pollutants in the atmosphere.  They can also be damaged by moisture.

Another example is amethyst which will fade if exposed to sunlight.

Clarity is taken into consideration when determining the value of certain gemstones.  It's not always relevant but depends on the type of stone.  A fine aquamarine should be almost transparent and void of inclusions yet it's rare to find a flawless emerald.

The term flawless is used to describe a gemstone that boasts complete transparency and is free from inclusions, cracks and anomalies.

Faceted Stones and Cabochons

A gemstone can either be polished as a cabochon or have facets cut into it.  The purpose of both is to maximise beauty which in turn increases value.

The term cabochon describes a gemstone that has been polished to produce a smooth rounded upper surface and flat base.  The technique is used mainly on stones that are opaque or exhibit minimal translucence.  The domed shape helps show off surface colour and markings as well as other characteristics such as chatoyance, schiller, asterism and play of colour.

Highly translucent or transparent gemstones tend to be faceted.  This involves cutting a series of flat reflective faces called facets into the surface.  This enhances the stone's ability to reflect and refract light.  In doing this colour, sparkle and brilliance can be enhanced.

Article Pictures

The pictures in our article are clickable and redirect to the original non-compressed image.  The first photo shows inclusions of black rutile in the mineral quartz.  

The banded amethyst stones in our second photo come from our collection.

The pencil in the glass of water demonstrates refraction of light.  The labradorite stones in our last photo exhibit the optical phenomenon of schiller or labradorescence.

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