What is a Gemstone?
1. Meaning of the Word Gemstone
2. Colour of a Gemstone
3 Optical Phenomena in Gemstones
4. Hardness Toughness Stability
5. Faceted Stones and Cabochons
6. Article Pictures
Meaning of the Word Gemstone
A gemstone is a section of a crystal, rock or mineral that has been cut and polished to create an individual stone.
Translucent gemstones such as ruby, topaz, peridot and aquamarine tend to be cut from a mineral's crystals. Opaque gemstones usually come from the actual rock or mineral. Examples include lapis lazuli, labradorite, bloodstone and jasper.
Stones classified as "gemstones" are valued for their beauty, rarity and to some degree durability.
The tradition of using certain gemstones as a birthstone can be traced back to Poland in the 18th century. The idea of a gemstone being associated with the month someone was born or with their zodiac sign can be traced back to the 1st century AD.
Evidence of our fascination with gemstones can be found throughout history. 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 this naturally occurring solid has been cut and polished.
Gemstones are mentioned in texts of many religions and have long been used in rituals and ceremonies.
In ancient Egypt they were used as part of the complex burial rituals of the pharaohs because they were believed 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, 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 classifying a gemstone. The three most important characteristics are beauty, durability and rarity but without beauty the other two mean very little.
What causes the value of one gemstone to increase can have the opposite effect on another.
Rubies, sapphires and emeralds are prized for their colour yet many diamonds are colourless.
The finest aquamarine can command an exceptional price if free from inclusions. Some varieties of quartz increase in value because of inclusions.
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.
Colour is determined by three main factors, the mineral's chemical composition, crystal structure and natural defects and inclusions.
The slightest change in crystal structure which could be due to variations in the chemical composition, growth conditions or other factors can bring about a change in colour.
This is why gemstones of the same mineral variety can be found in a wide range of colours.
A particular characteristic that causes a change in colour may always be the same or can differ depending on impurities. An impurity can also influence the amount of light that's absorbed or reflected.
The largest group of gemstones are those which are mostly colourless in their purest form. These stones tend to be translucent or transparent.
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 light fades so does colour. Our eyes then only see black and grey.
A fine grade gemstone must exhibit good depth of colour without being too pale or too dark. Colour should be uniform throughout which in some gemstones can be an issue.
In amethyst the shade of colour often varies within the same crystal. This characteristic known as colour zoning may be subtle or prominent.
Banded amethyst (also known as chevron amethyst) is the most extreme example of colour zoning.
Optical Phenomena in Gemstones
Many gemstones exhibit optical properties. These characteristics are also described as optical phenomena. They can often lead to a significant increase in value.
Optical phenomena are often caused by the way light interacts with the stone or 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.
Lustre should not be confused with brilliance which is the reflection of light from within a cut gemstone.
Terms used to describe optical phenomena 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 silica spheres. This causes 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.
Adularescence is often used to describe the same effect in moonstone. As light reflects off different layers a milky-white glow or blue sheen can be seen.
"Aventurescence" can be seen in aventurine and sunstone. Asterism produces a four or six rayed star in some cabochon-shaped gemstones. It's caused as light reflects off crystal inclusions of rutile.
The chatoyance 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 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 the same but are different.
Hardness relates to scratch resistance meaning how easily one gemstone can be scratched by another. Mohs scale of mineral hardness is widely used for this purpose.
Toughness relates to a gemstone's resistance to cracking, chipping, pressure or breaking. It can also affect how easy it is to facet.
Whilst diamond may be the hardest mineral it's not the toughest. Diamond grades 10/10 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 on Mohs scale. Despite its hardness it's fragile and can be easily chipped. 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 reference Mohs scale of mineral hardness. Mohs scale is used to determine hardness or scratch resistance not toughness.
The only way of finding information about toughness 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 as an example, this mineral is delicate and porous so can be easily damaged by pollutants in the atmosphere. It 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 can be almost transparent and void of inclusions yet it's incredibly rare to find a flawless emerald.
Flawless describes a gemstone that boasts complete transparency and is free from inclusions, cracks and anomalies.
Faceted Stones and Cabochons
A gemstone can be polished as a cabochon or have "facets" cut into it. The purpose of both is to maximise beauty which increases value.
A cabochon is a gemstone that's 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 of a cabochon enhances 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 a gemstone's ability to reflect light which improves colour, sparkle and brilliance.
Several of the pictures in our article are clickable and redirect to the original images. The first photo is black rutile inclusions in quartz.
The banded amethyst stones are from our collection. The labradorite stones exhibit the optical phenomenon of schiller or labradorescence.
The final photo is a colourless topaz crystal. The topaz and rainbow photos are courtesy of Stan Celestian.