Choosing a Gemstone

How to Choose and Evaluate

In the mid-20th century, the Gemmological Institute of America (GIA) introduced their method of describing Diamonds – the Four C’s.  The Four C’s follow four categories: Colour, Clarity, Cut and Carat; however, this can technically be applied to all gemstones not just Diamonds.

All gemstones have colour – some are pale, others are bright and vivid.  You might find black or completely colourless (white) stones too!  For many varieties of gemstones, colour intensity is one of the primary factors in determining their value.

As with colour, all gemstones have clarity.  This describes how clean the gemstone is.  Inclusions (flaws) usually lower this grading.  For Diamonds, this is one of the characteristics which is at the forefront of attention.  The clarity grading scale ranges from F to I.  Within these grades (excluding F or IF) is a series of sub-gradings ranging from 1 (best) to 3 (worst).


Flawless, no inclusions evident anywhere, inside or outside.


Internally Flawless, no inclusions inside the stone.


Very Very Slight Inclusions, a microscope is needed to spot these inclusions.


Very Slight Inclusions, a hand loupe is needed to spot these inclusions.


Slight Inclusions, these inclusions are barely visible with the Human Eye.


Included, these inclusions are easily visible with the Human Eye.


Cut is another category, this entails the fashioning and shape of the stone.  Was it cut for weight or for presentation?  Cut also describes the shape of the stone.  Additional weight in gemstones is the usual way to add value, however, in some gemstones, having an excellent cut can add more value over a slightly heavier stone.  Some gemstone shapes are quite common relative to the variety of gemstone they are from, whilst others are quite rare; this can increase the value of a gemstone purely based upon the rarity of that cut.

Carat is the final category.  As touched on in Cut, Carat weight is the characteristic for the size of the gemstone, generally a heavier stone is worth more than a lighter gemstone, however in some cases, a smaller, lighter gemstone, with higher grades in Colour, Clarity and Cut can still possess a higher value than a larger, heavier gemstone.


There are many different types of gemstone treatments, some of these are highly invasive and others are less, and more accepted in the marketplace.  Most treatments are conducted to improve the colour or clarity of the gemstone.

Heat Treatment, is a method of enhancement where the gemstone material is placed within a furnace and subjected to high levels of heat in order to enhance the clarity or colours of the original gemstone.  This is a widely practiced treatment and is readily accepted in the global marketplace.

Diffusion Enhancement, this is a form of gemstone enhancement where a foreign element is introduced to the material and diffuses its way into the stone and changes the colour.  The easiest way to think of how this works is that the stone soaks up the element and in turn changes its colour based upon that element.

Irradiation Enhancement, similar to Diffusion Enhancement, a gemstone is placed within a small nuclear reactor and bombarded with energy in order to force certain colour combinations not normally attainable in nature.  Don’t worry, there is no threat of radiation poisoning from these treatments, as the methods used today require the stone to be radioactively inert before introduction to the marketplace.

Oil Treatments are used when the host material has many surface reaching inclusions which need to be filled in order to enhance the clarity of a gemstone.  There are three types of Oil Treatments.  Colourless Oil without a hardener, Colourless Oil with a hardener, Coloured Oil with a hardener.  Colourless Oil without a hardener is the preferred treatment, as the Oil can be extracted, if so desired.  Oils with hardeners prevent this extraction process, and Coloured Oils add an artificial colour to the stone, akin to painting it another colour.

Some gemstones are filled with Glass in order to fix large inclusions as a way of improving clarity, however this often seriously lowers their value, and is not highly regarded in the marketplace.

Finally, we have the Natural or No Treatments category.  Depending on the variety of gemstones, this could be a relatively common occurrence, or quite a rare occasion.  For some gemstones, the mere fact that the stone has been untreated since its original discovery, means that the stone gains a huge amount of value for this fact alone!



Though a lesser factor for some stones, knowing the Origin of a gemstone is often a huge boon, especially if that Origin is a highly regarded one.  In some cases, an Untreated stone with a specific Origin holds immense value over a similar stone with some form of treatment and unknown origin.



Lab made gemstones are known as Synthetics.  These gemstones hold little value and should always be disclosed whenever possible.