What Exactly Are Gemstones?

collection of colourful faceted gemstones

 

Gemstones Earth's Natural Treasures


Man's fascination with gemstones stretches back to the dawn of time and is common to every age and every culture.  We're captivated by their properties, their uses, the way they react to light and the transformation that takes place once a stone has been cut and polished.  Our relationship with these inanimate objects is more than just about beauty, throughout history there's been a belief that gemstones hold magical powers and healing properties with 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, 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.

Although several factors are taken into account when determining the suitability of a gemstone, the three most important 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 sometimes have the opposite effect when present in a different type of stone.  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


Black Rutile Inclusions in Quartz

 

 

Optical Phenomena and Colour 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 or other physical effects.  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 be an issue particularly with amethyst whose colour 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 an 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 band of light reflects off crystals, fine parallel 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 stones 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 can produce incredible flashes of colour whilst aventurescence describes the optical reflectance in sunstone and aventurine quartz.  The effect is like a metallic glitter caused by minute plate-like inclusions which if present in great abundance can change the colour of the stone.

The one thing that all optical effects 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.  Furthermore, without light there would also 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.  The pearly lustre seen in pearls is caused by reflections from parallel layers of transparent materials.  The polishing of a material will almost always improve lustre.  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 crystal structure can have a significant impact.  In jasper inclusions of hematite produce a deep shade of red whilst the copper in malachite 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.  Zinc introduces hues of yellow.

With the presence of iron 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 (Adularescence) in Labradorite

 

 

Hardness Toughness Stability


Durability is another factor that determines the suitability of a crystal rock or mineral to be classified as a gemstone and 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 gemstone can be scratched by another.  Mohs scale of mineral hardness is a tool widely used for this purpose.  Toughness relates to a gemstone's resistance to cracking, chipping or breaking and affects how easy it is to facet.  Whilst diamond may be the hardest stone on the scale it's certainly not the toughest.  Diamond grades 10 whilst nephrite jade grades 6 to 6.5 yet is considerably tougher 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.

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.  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 often 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. Gemstones which are highly translucent or transparent tend to be faceted which involves cutting a series of flat reflective faces called facets into the surface of the stone.  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 cut and polished, a dramatic transformation takes place.  The vast majority of gemstones are cut from minerals or crystals.  One exception is lapis lazuli which is a rock that's made up of several different minerals.

 

 

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