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What Exactly are Gemstones?

 

colourful faceted gemstones

 

Contents

1. Gemstones Earth's Natural Treasures
2. Colour and Optical Phenomena in Gemstones
3. Hardness Toughness Stability
4. Faceted Gemstones and Cabochons
5. Where Do Gemstones Come From?
6. 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 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 stones 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 completely colourless.  Gem grade aquamarine and topaz command exceptional prices when free from inclusions yet some types of quartz increase in value because of their inclusions.  

 

 

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


Inclusions in of rutile in quartz

 

 

Colour and Optical Phenomena in Gemstones


The colour of a mineral which can be its most important feature can also be one of the least reliable for correct identification.  Although crystals rocks and minerals occur in a wide range of colours, many have been transformed because of minute impurities, other physical effects or through heat treatments.  Identification by colour alone has led to many gemstones being incorrectly identified.

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.  The colour amethyst which is the purple variety of the mineral quartz can vary within the same crystal.  This natural anomaly is known as colour zoning and chevron amethyst is the most extreme example.

 

 

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

 

 

Some gemstones exhibit fascinating optical phenomena and this quality alone can lead to a significant increase in value.  The effects include but are not limited to chatoyance, play of colour, iridescence and asterism.

Chatoyance is caused as a parallel band of light reflects off crystals, inclusions or fibres within the stone and is best known for its presence in the minerals chrysoberyl and tigers eye.  Asterism which produces a four or six rayed star in certain gemstones when shaped as a cabochon is caused as light reflects off inclusions of the mineral rutile.  Iridescence which can be seen as 'play of colour' in precious opal appears as light scatters across the surface of the stone.  The sheen in labradorite and moonstone which is correctly known as schiller or adularescence can produce incredible flashes of colour whilst aventurescence describes describes a slightly different optical reflectance that can be seen in the minerals sunstone and aventurine quartz.

The one thing that all optical phenomena have in common is that they're caused because of the way that light interacts with the material's crystalline structure.  The extent to which they can be seen is usually dependant on the angle of light and angle of observation.  Interestingly without the presence of light there would be no colour because we only see colour because of the way that 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 colorful when light which is energy hits them but as it fades, so does the colour and our eyes then only see black and grey.

The word lustre is used to describe the way that light interacts with the surface of a crystal rock or mineral.  Whilst vitreous is the most widely used in reference to gemstones, others include dull, metallic and adamantine.  By polishing a gemstone its lustre will almost always improve.  Lustre should not be confused with brilliance which the reflection of light from within a gemstone that has been faceted.

 

 

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

 

 

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 differ depending on the constituents present.  The tiniest alteration in the stone's crystal structure can have a significant impact.  In jasper inclusions of hematite produce a deep shade of red whilst in malachite the presence of copper is the cause of the stone's green colour.  The varying shades of blue in turquoise are caused by copper and aluminium but when aluminium is replaced with iron the colour changes to green.  When zinc is present in turquoise the gemstone exhibits hues of yellow.

With the presence of iron the mineral corundum turns yellow, with the addition of titanium it turns blue and with chromium it turns red.  Only red corundum is known as ruby whilst all other colours are known as sapphire.

In its purest form just like corundum the mineral beryl is colourless but traces of other minerals cause different shades of red, green, yellow and blue.  Green beryl is emerald whilst blue is aquamarine.  Other contributing factors responsible for colour change in rocks and minerals 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.

 

 

collection of different sized labradorite cabochons isolated on a white background

Schiller (aka adularescence or labradorescence) in labradorite

 

 

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 in fact 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 certainly not the toughest.  Diamond grades 10 on Mohs scale whilst nephrite jade grades 6 to 6.5 yet is a much tougher material 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 and or gemstones all of the search results direct towards 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 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 also 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 for example 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 usually 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 a flat base.  The technique is used mainly on rocks and minerals that are opaque or translucent. The domed shape helps to 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 gemstone's ability to reflect 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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