Precious and Semi Precious Stones. Part A: What's the difference?


In the wonderful world of gemstones, most are divided along the lines of Precious and Semi-Precious stones.  But what does this mean and how can you tell the difference?

Diamonds, Sapphires, Rubies and Emeralds are the precious stones, but why? What makes them different from the semi-precious stones?


Some rough sapphires, rubies and emeralds in my collection


For a start, it's their hardness.  There is a scale of hardness for stones known as the Mohs scale of mineral harness.  Top of the scale at 10 are diamonds. Coming in at 9 are the corundums, including sapphire and rubies.  These are much the same, corundum having two primary gem varieties: ruby and sapphire. Rubies are red due to the presence of chromium, and sapphires show a wide range of colors depending on what metals are present. Emerald is a beryl and has a hardness of around 8.

In all gemstones, but particularly the precious ones, what's important in assessing the value of the stone is 'the four C's', colour, clarity, cut and carat (or weight).

All other stones such as amethyst, topaz, aquamarine, garnet, peridot etc are known as semi-precious and most have a hardness of between 6 and 8. Turquoise, amber etc are also classed as semi-precious but have a much lower Mohs rating.

The Gemalogical Institute of America (GIA) has a series of scales against which all precious stones are rated.  

In diamonds for example, Colour ranges from D through Z where D is colourless (and more highly valued) and Z is a mid-yellow.  The diamonds I use are often Z and beyond, as I like the odd-coloured diamonds in yellows and ambers.

Some fancy coloured diamonds showing the range of colours


Clarity ranges from flawless to included (meaning you can see some inclusions in it)  most of the diamonds I work with are fairly heavily included and do not rate anywhere on the GIA Clarity Scale. Nearly all emeralds are included - some heavily.


A paler green, moderately included emerald of 1.67 ct

I actually rather like inclusions in gemstones, as it shows a little more of the character and individuality of how they were formed.

Cut refers to more than just the shape of the stone, but how the cut shows off the special qualities of that particular stone.

Carat is the weight, where 1 carat is 0.2 of a gram.  Most of the stones I use are around 0.2 carat to 0.5 carat.

Rough diamonds are a whole different ballgame. They tend to be square in shape and completely opaque.  Some jewellers such as Todd Reed have developed a whole line of jewellery using these.  I have a few which I intend to set in the near future.

(a) Todd Reed Ring.   (b) Some of my rough diamonds

 The boundary between gem-quality diamonds and industrial diamonds is poorly defined.  While the diamonds I use are not gem quality, neither would I class them as 'industrial'.

Some of my fancy coloured diamonds

So far we have concentrated mostly on diamonds, but the other precious stones are equally fascinating and deserve our attention.  

In my next Blog we will talk about common treatments and enhancements for precious stones.  Hang in there - it's coming soon.