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Gemstones | What Exactly are They?

 

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 and today our bond with these magical 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 a gemstone has been cut and polished.

For many people the relationship with a gemstone is more than just about beauty.  Throughout history it's been believed that the rocks and minerals from which gemstones are cut hold healing properties and magical powers that have the potential to influence mental and physical behaviour.  They're mentioned in the sacred 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 in the the belief they 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 quite common for people to be buried with their most prized possessions and these items have proven to be 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 of which 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 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 and the same characteristic can even have the opposite effect when present in a different gemstone.  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 their 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 but it's also the least reliable for accurate identification.  Identifying a stone based on colour alone has led to many mistakes being made.

A fine grade gemstone must exhibit good depth of colour without being too pale or too dark.  The colour should ideally be uniform throughout the stone which can often be an issue.  In amethyst the shade of colour can vary within the same crystal.  This natural characteristic is known as colour zoning and whilst it can be very subtle in some gemstones, it can also be quite extreme.  Chevron amethyst is the most extreme example of colour zoning.

 

 

seven large sized chevron amethyst tumbled stones
Chevron amethyst the most extreme example of colour zoning

 

 

Colour in gemstones is caused by the absorption or refraction of light or in other words, the way a gemstone reacts when in contact with light.  White light (light that appears to be colourless 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 of which 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 within the object then speed up again once they come out.  When light changes speed it also 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 the 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 so 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 the pencil enters the water it looks to be misaligned which is caused by refraction of light. The light only bends at the surface but 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 and 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 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 it becomes amethyst.  Corundum turns red with impurities of chromium but replace chromium with iron and it turns yellow.  With titanium and vanadium it turns blue.  Red corundum is ruby whilst all other colours are sapphire.

In jasper inclusions of hematite produce a deep shade of red whilst in malachite the presence of copper causes the stone to be green.  The varying shades of blue in turquoise are caused by copper and aluminium but when aluminium is replaced with iron the colour is more green.  With zinc hues of yellow can be seen.  In its purest form the mineral beryl is colourless but different impurities cause shades of red, green, yellow and blue.  Green beryl is emerald, blue is aquamarine.

Other contributing factors responsible for colour change in minerals and gemstones include heat, the orientation of the crystals and the presence of a structural imperfection such as 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 light which is energy hits them but as it fades, so does the colour and our eyes then only see black and grey.

 

 

Optical Phenomena in Gemstones


Many translucent gemstones exhibit fascinating optical phenomena and this quality alone can lead to a significant increase in value.  The different effects that can be seen are caused either by the reflection or refraction of light and in some cases by both.  Light may be reflected from 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 widely used in reference 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.

Several terms are used to describe different optical phenomena in gemstones some of which include 'play of colour', adularescence, schiller, labradorescence, aventurescence, chatoyancy 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 then reflects off different layers.  The colours we see are caused by the reflections.  Although correctly known as schiller the term labradorescence was coined because the mineral 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 within the stone a milky white glow or distinctive blue sheen can be seen.  Aventurescence is another optical phenomenon seen in aventurine quartz 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 or chatoyancy best known for its presence 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 parallel band of light moves giving the impression of movement on the surface 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 rock or mineral can be scratched by another.  Mohs scale of mineral hardness is a tool that's widely used for this purpose.

Toughness relates to a rock or mineral's resistance to cracking, chipping, pressure or breaking and with regards to a gemstone 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 yet it's much tougher because of its strong interlocking network of fibrous crystals.  Pearl grades just 3 on the scale yet is extremely tough and if dropped is unlikely to break.  Drop a diamond and the consequences will probably be very different.  Gold is another soft mineral on Mohs scale yet if you pound it with a hammer it will just get flatter and flatter. 

When searching for information online regarding toughness in relation to a rocks minerals or gemstones all of the search results direct towards Mohs scale of mineral hardness.  The only way to find accurate information about this property 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 are delicate and porous hence can easily be damaged by pollutants in the atmosphere and moisture.  Another example is amethyst which is prone to fading if exposed to bright sunlight for long periods of time.

Clarity is taken into consideration when determining the value of certain gemstones but may not always be as relevant depending on the type of stone.  A fine aquamarine should be almost completely transparent and void of inclusions yet it's extremely 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


Gemstones are either polished as cabochons or have facets cut into them and 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 gemstones that are either opaque or exhibit minimal translucency.  The domed shape helps show off surface colour and markings as well as other characteristics such as chatoyancy, 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 gemstone's ability to reflect and refract light which improves colour, sparkle and brilliance hence making it considerably 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 full size image.

 

 

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