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What Exactly are Gemstones?

 

 

colourful faceted gemstones

 

Contents

1. Gemstones Earth's Natural Treasures
2. Colours Present in Gemstones
3. Optical Phenomena in Gemstones
4. Hardness Toughness Stability
5. Faceted Gemstones and Cabochons
6. Where Do Gemstones Come From?
7. Articles Photos

 

 

 

Gemstones Earth's Natural Treasures


Man's fascination with gemstones dates back thousands of years.  Today our bond with these natural curiosities is stronger than ever.  Common to every age and every 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 a gemstone is more than just about beauty.  Throughout history man has believed the rocks and minerals from which they're cut hold curious powers and healing properties.  They're believed to have the ability to influence mental and physical behaviour, are mentioned in the texts of many 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.  They believed this revered mineral would ensure immortality.

In ancient Egypt gemstones were used as part of the complex burial rituals of the pharaohs 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 Museum of Natural History in Washington D.C, The Armoury Palace in Moscow and the Tower of London are home to some of the largest and most beautiful stones 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.  Furthermore the same characteristic can have the opposite effect when present in a different stone.

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

 

 

black needle-like inclusions of the mineral rutile in a marquise shaped quartz gemstone


Inclusions of rutile in quartz

 

 

Colours Present in Gemstones


The colour of a mineral or gemstone can be its most important feature.  It can also be the least reliable for accurate identification.  Identifying a stone 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 can vary within the same crystal.  This characteristic known as colour zoning can be subtle or clearly visible.

The variety known as chevron amethyst (banded amethyst in the UK) is the most extreme example of colour zoning.

 

 

seven large sized chevron amethyst tumbled stones
Chevron amethyst from our collection

 

 

Colour in gemstones is caused by the absorption or refraction of light.  This basically means the way the surface interacts with light.  White light (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 the 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 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.  A good example is a pencil in a glass of water.  At the point where it makes contact with the water the pencil 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 (raindrop) is also round.

 

 

pencil in a glass of water demonstrating refraction of light


 An example of refraction of light 

 

 

The largest group of gemstones are those which are mostly colourless in their purest form.  These stones usually exhibit a high degree of translucency.  The characteristic that causes colour to change 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 the presence of iron.  The colour of rhodochrosite is caused by manganese.  In its purest form quartz is colourless but with the presence of iron and exposure to heat it transforms into amethyst.

Corundum turns red with impurities of chromium.  Replace the chromium with iron and it turns yellow.  With titanium and vanadium corundum turns blue.  Red corundum is ruby, all other colours are sapphire.

In the stone known as jasper inclusions of hematite produce a deep shade of red.  The green colour in malachite is caused by the presence of copper.  The varying shades of blue in turquoise are from copper and aluminium.  When iron replaces aluminium the colour is more green.  With the presence of zinc hues of yellow can be seen.

In its purest form the mineral beryl is colourless but various impurities cause shades of red, green, yellow and blue.  Green beryl is emerald, blue is aquamarine.

Other contributing factors responsible for colour change in rocks minerals and gemstones include heat, the orientation of the crystals and the presence of a structural imperfection.  This could be some kind of damage or any 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 light which is energy hits them but as it fades, so does the colour.  Our eyes then only see black and grey.

 

 

Optical Phenomena in Gemstones


Many gemstones exhibit fascinating optical phenomena.  This quality alone can lead to a significant increase in value.  The effects that can be seen are caused either by the reflection or refraction of light and in some cases by both.  Light may reflect off the surface of a stone or off inclusions beneath the surface.

The word lustre is used to describe the way light reflects off the surface of a rock mineral or gemstone.  Although the type of lustre known as vitreous tends to be the most common in respect to gemstones, there are several others.  By polishing a gemstone its lustre will almost always improve.  Lustre should not be confused with brilliance which is the reflection of light from within a gemstone that has been faceted.

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 the silica spheres.  The result is a spectrum of colours.

The iridescence that can be seen 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

Schiller aka labradorescence in labradorite

 

 

The word adularescence is often used to describe the effect when seen 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 the minerals aventurine and sunstone.  Asterism produces a four or six rayed star in some gemstones when shaped as a cabochon.  The effect is caused as light reflects off inclusions of the mineral rutile.  

Chatoyance often seen in tigers eye and chrysoberyl is caused as a parallel band of light reflects off thin parallel 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 the effect can be seen is dependant on the angle of light and angle from which the mineral or gemstone is being observed.

 

 

opal with distinctive play of colour. Museum exhibit on a black background
Opal from Andamooka Southern Australia. Courtesy of Stan Celestian

 

 

Hardness Toughness and 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 one and the same but are quite different.

Hardness relates to scratch resistance meaning how easily one stone can be scratched by another.  Mohs scale of mineral hardness is a tool 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 material 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 just 3 on Mohs scale yet is extremely tough and if dropped is unlikely to break.  Drop a diamond from the same height and the consequences are likely to be very different.

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 to find accurate 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 an example, both 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 is prone to fading if exposed to sunlight for long periods of time.

Clarity is taken into consideration when determining the value of certain gemstones.  It may not always be quite as relevant depending on the type of stone.  A fine aquamarine should be almost completely 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.

The weight and size of a gemstone is also relevant however where large crystals are in abundance such as in quartz and topaz, the increase in price will be considerably less.

 

 

Faceted Gemstones and Cabochons


A gemstone can be polished as a cabochon or have facets cut into it.  The purpose of both is to maximise beauty.

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 either 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 practice dramatically enhances the stone's ability to reflect and refract light.  This improves colour, sparkle and brilliance which makes it more beautiful which in turn increases value.

 

 

Where Do Gemstones Come From?


Gemstones come from rocks and minerals but not all rocks and minerals produce gemstones.

Although found in all three rock forming environments, gemstones form primarily through igneous and metamorphic processes.  Uncut stones can often look ordinary and uninteresting but once faceted and polished a dramatic transformation takes place.

 

 

Article Photos


The photos in our article are clickable and redirect to the original non-compressed image.

 

 

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