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Gemstones are a huge subject, so I will present you with a few facts here that I hope will both help you to identify with the subject and maybe sharpen your appetite to look for more information.

There are some very good, readable books on the subject of gemstones and if you are thinking of buying a fairly expensive stone then I would encourage you to do a little reading before buying.
 
 
  Learn how to assess a gemstone’s value
  Clarity or Transparency
  The Cut
  The Colour
  Carat Weight
  Fake Gemstones
  Imitation Stones
  Composite Stones
  Synthetic Stones
  Treatments
  Star Rubies and Star Sapphires
  Fake Star Stones
  Diamonds
  Emeralds
 
 
 
 
LEARN HOW TO ASSESS A GEMTSONE'S VALUE:
When jewellers talk about the quality of a stone, and precious stones in particular (being mainly diamonds, sapphires, rubies and emeralds), they often use four main criteria to make a value judgement.
 
These are Clarity, Cut, Colour, and Carat weight of the stone in question.
 
CLARITY OR TRANSPARENCY:
Clarity or transparency refers to both the marks and blemishes on the surface of a stone and also the inclusions (natural markings on the inside of gemstones) that came about when the stone was created.
 
Generally speaking, the better the clarity, and the fewer the marks on or in the stone, the more valuable it is. However, a perfectly clear precious gemstone can be treated with a little suspicion and if it is expensive, it should be tested in a gems laboratory to find out if it is natural. It costs me $15 to get a basic report on an expensive stone by the Asian Institute of Gemmological Sciences in Bangkok. This allows me to sell such stones with confidence, and with a report.
 
A natural stone will very often have some marks or inclusions which identify it as being natural and genuine. Among the precious stones, inclusions come in varying types and quantities associated with that type of stone. For example Sapphires have fewer inclusions generally than Rubies, and Rubies have far less inclusions than Emeralds.
 
Inclusions (natural markings on the inside of gemstones) that are clearly visible to the eye are very common in expensive emeralds, and form part of their attraction. An emerald with no inclusions or marks should be treated with a great deal of suspicion.
 
A very fine Ruby however may be virtually clear of visible inclusions, and you can expect good Sapphires to be mostly clear of such eye-visible natural markings.
 
However, the presence of marks and inclusions do give the stone its own character and can be proof of a natural birth. Do not be afraid to buy a stone with inclusions; you should be more worried about one that has none! If it is very clear, it may be a man-made synthetic stone.
 
It is only when inclusions in a ‘precious stone’ are very visible to the naked eye and really detract from its appearance that you may consider whether that is the right stone for you. Sometimes a smaller but clearer stone (maybe costing the same or a little more) is the better option.
 
Inspecting stones with a ten power loupe, or magnifying lens, is the ideal way to check the clarity.
 
Clarity is more delightful to the eye than even good colour.
 
If a stone is opaque and has surface flaws (bad clarity), but has a good colour, it will not be so pleasing as a clear stone with a less desirable colour.
 
THE CUT:
The cut of a stone is a demonstration of the cutters skill at bringing out the brilliance of the stone from the rough.
 
If making money is the main aim of the cutter he will make the face of the stone as big as possible at the expense of the correct proportions required for brilliance and ‘life’ in the stone.
 
So the meaning of ‘cut’ here is creating the correct proportions of the various parts of the stone to give ‘life’ to it. For example, the refractive index of a Sapphire requires that the depth of the stone, from top to bottom, should be 60 – 80% of the diameter.
 
This means that if it is cut with the correct number of facets, the light will all reflect back up to the observer so you should not be able to see right through the stone, and read newsprint for example. If you can see through it, this is called ‘windowing’. This kind of defect in cutting will drastically affect the price even if the colour and clarity are good.
 
Be aware that if a stone is cut much deeper than necessary, you will be paying for it by carat weight, and this stone too was cut to maximise its selling price.
 
THE COLOUR:
The colour of precious stones is a great determinant in their value.
 
Pale colours are generally considered to be less valuable than medium to dark colours.
 
Another aspect to Colour is when there is a presence of a brown or grey hue.
 
So if the blue sapphire you are looking at has a discernable element of grey, or the ruby has an obvious brown hue present, their value should be considerably less than a stone with ‘pure’, ‘strong’, ‘saturated’, ‘intense’, ‘vivid’ colours. These are all acceptable words to describe this high quality condition.
 
The optimum colour for rubies should demonstrate a red hue, medium to medium dark colour and no visible brown colour.
 
Blue sapphires are the most expensive and popular of the sapphire family. Blue sapphires come from the same mineral group as Rubies, and the family is called ‘Corundum’. Chemically speaking, ‘Corundum’ is Aluminium Oxide and the red colour of Rubies is created by the addition of Chromium, and the blue colour of Blue sapphires is created by Iron and Titanium. Other colours, or ‘Fancy sapphires’, are created by a combination of these chemicals.
 
The most prized blue sapphires are medium to medium dark blue, which is hard to imagine without a comparison. Look at as many blue sapphires as possible to build up a ‘comparison chart’ in your memory.
 
If the stone is nearly black, it is less valuable than a good strong visible blue under the lighting conditions in which it will be worn. So if you expect your lady or the wearer to carry the stone at evening dinner parties for example, consider what the colour will look like in low light conditions. I prefer a lighter colour blue, cornflower blue maybe, for this reason.
 
All this is especially true where emeralds are concerned. Valuable emeralds should not have visible hues of brown or grey. Again, the best ones are claimed to be intense dark green colours, but not blackish.
 
Emerald is a mixture of bluish green and yellowish green colours, both of which may be visible in different conditions.
 
CARAT WEIGHT:
The effect of carat weight on the value of precious stones is dramatic.
 
You would be forgiven for thinking that a five carat good quality blue sapphire would be five times the cost of a one carat similar quality stone. Sadly this is not true.
 
The cost ‘per carat’ of the 5 carat stone may be many times the ‘per carat’ cost of the smaller one. To find out the ‘per carat’ cost of the stone, divide the cost of the stone by the carat weight.
 
Gemstones are priced ‘per carat’, and the prices are directly to do with supply. Simply, the larger stones are much rarer and so the ‘per carat’ price is much higher.
 
The specific gravity of the stone has an interesting effect here. Emerald has a lesser specific gravity than rubies, sapphires and diamonds, and consequently a one carat emerald may look twice the size of the others. This may sound like a good reason to buy emeralds, but the quality of emeralds is so variable that good quality stones are still very expensive, even if they are small.
 
Look at any stone to see where the weight has been put in the cutting. If it is very deep cut, you will be paying for a lot of stone that you will not be looking at.
 
But bear in mind the ‘correct’ proportions mentioned above in ‘Cutting’ and the relative ‘life’ or ‘brilliance’ of the stone in question. Look and see if it is a very ‘shallow’ cut so that all the weight is in the front, or table. This will make the stone look very big for its weight, but how much windowing is there as a result and is there a lack of ‘life’ and ‘brilliance’?
 
The distribution of the weight is therefore also critical to the value of a stone.
 
Incidentally, in choosing the size of a stone for a ring, consider the size of the finger, be it for a man or lady. The stone needs to look good on the finger and should be of the correct proportion.
 
For example, a stone that is too small on a mans finger may look odd. And a stone that is way too big on a ladies finger may look equally out of proportion.
 
Where a budget is involved, a small fiery stone may be a better choice than a large dull one.
 
FAKE GEMSTONES:
The meaning of fake here is gemstones that are sold as natural, when in fact they are imitation stones or man made synthetic stones, also undisclosed, unapproved treatments, and also composite stones.
 
The key here is if they are fraudulently sold as being a precious stone, and their real origin is undisclosed. All these are legal if there is proper and full disclosure when they are sold.
 
A brief explanation of each type follows:
 
IMITATION STONES:
Imitation stones are those created to look like what they are being sold as, but are not real.
 
For example, red glass as ruby, or colour varnished quartz. These are often used in antique jewellery, with maybe only one or two fake stones in a piece containing many stones. Often these are foil backed or made in a closed back setting. They may also have a coloured varnish or other coating on the back.
 
Natural stones are always set with an open back or with the pavilion (back of the stone) showing.
 
Good indications of a suspect stone are:
The price: is it being sold way too cheap?
Is it completely clear of marks or inclusions?
Look closely at the stone; does it look like glass with rounded facet edges instead of crisply cut ones?
 
Consider getting a lab report if you are in any doubt about a valuable stone.
 
COMPOSITE STONES:
Composite stones are stones made up of layers of less expensive material glued together to look like the real thing.
 
Composite stones are either DOUBLETS, with 2 layers of stone or TRIPLETS, with 3 layers. It can be difficult to spot this type of stone.
 
But if you hold the stone under water (in a glass) and look at it under magnification (as you should at all stones) from the side, you may see a joining line. Often the different layers may be visible, they may also be clear or coloured. Coloured glues between the layers have also been used to ‘improve’ the look of the stone.
 
This type of fake can produce an exceptional look-alike at a small cost of stone, where labour is cheap.
 
Labour is cheap where most gemstones are mined.
 
SYNTHETIC STONES:
Synthetic stones are lab grown or man made.
 
There is a huge market for synthetic stones in the modern world, genuinely sold mainly under generic names.
 
Emerald: There are two types of synthetic emerald; hydrothermal and flux grown. These are both chemically made in solutions and involve crystallization.
 
Sapphire: Synthetic sapphire has been available since the early 20th Century, and is common in all types of jewellery found today.
 
Flame fusion sapphire is made by melting powdered chemicals and allowing them to crystallize. Melt pulled sapphire is commonly used in industrial applications. Flux grown sapphire is made by the same process as flux grown emerald. Hydrothermal sapphires likewise are made by the same method as hydrothermal emerald
 
Many flux and hydrothermal grown stones are now produced in Russia and are widely sold as generic creations.
 
TREATMENTS:
Heat treatment of stones is an accepted practice in the trade because it leads to a permanent improvement of the colour the stone, and it is too widespread to condemn or stop.
 
Blue sapphires and rubies that have not been heat treated are called ‘natural’ by the trade and are now sold at a considerable premium.
 
Sapphires and rubies that are sold as ‘natural’ or unheated from Myanmar (Burma), Thailand or most Asian countries, including Australia are also sold for greatly inflated prices ‘per carat’ because they are so rare.
 
These ‘natural’ stones are already sufficiently ‘saturated’ with vivid colour and the fact that they are not heat treated is an additional rarity and value factor.
 
STAR RUBIES AND STAR SAPPHIRES:
Star gemstones are getting more difficult to find these days.
 
Star Rubies and Sapphires occur naturally when ‘silk’ or ‘rutile needles’ (needle like crystals) are present in the make up of the natural corundum.
 
The light reflecting off the cross hatched crystals creates a six-rayed star which moves around the face of the stone as you or the light source move around.
 
Naturally occurring stones are getting rarer due to the pervasive habit of heat treating corundum to ‘improve’ or deepen its colour and hence its value.
 
Heat treatment of this nature is often to above 1700 degrees Centigrade, and this dissolves the crystalline rutile needles, thus destroying the chances of a star stone.
 
Occasionally 12 rayed stars are found and this is a result of a blend of mineral ingredients.
 
Star Rubies and Blue Star Sapphires are the most well known and most sought after.
 
Black Star Sapphires have white rays or gold rays and are mainly found in Thailand.
 
The majority of Indian star rubies tend to be opaque and cracked on the surface, and are subsequently cheaper.
 
The best stars are transparent or translucent, which makes them much more valuable than opaque stones.
 
Star Rubies and Sapphires are highly valued and considered ‘lucky’ stones all over Asia.
 
Certainly you should expect to pay quite a lot of money for a good example and the pleasure of owning such a rare and beautiful stone.
 
FAKE STAR STONES:
The general features of a fake star stone are:
 
Profile: A flat base.
Clarity: Opaque.
Star: A too perfect star with long lines round the edge of the stone.
Number of rays: Sapphire and ruby does not have four rayed stars, only six or twelve rays.
Colour: A vivid red or vivid blue colour stone.
Setting: The back of the stone is closed to view.
 
If you come across a ruby or sapphire star stone being sold with the above features, you should be suspicious and request a report from a lab of your own choosing, not the sellers. Better still, walk away!
 
DIAMONDS:
Diamonds have a set of descriptive rules entirely to themselves, but it is enough to say that they should be bought from a reputable source only.
 
There are cheap imitations of diamonds as well as very good imitations that can fool professionals sometimes.
 
Cubic Zirconia is a stone that may be passed off as a diamond but it is fairly easy for a professional to distinguish between it and diamond. Cubic Zirconia is sold in its own right as a beautiful stone in less expensive jewellery.
 
In terms of similarity to diamond, man made Moissanite is so similar that a laboratory or special tools are needed to tell the difference. Moissanite is sold as the best man made copy of a natural diamond in existence, and you should expect it to be considerably less expensive than a natural diamond.
 
There are other similar looking stones, such as white sapphire, which can be substituted for diamond, so there are many reasons for dealing with traders you can trust.
 
Throughout the genuine trade, these diamond look-alikes are marketed openly under their generic names.
 
The risk of fraud mainly exists when someone tries to purchase stones away from a reliable supplier.
 
EMERALDS:
Emerald is a member of the Beryl mineral family, and so is Aquamarine. In fact a blue/green Aquamarine is considered by some to be a light green emerald.
 
Some of the most prized emeralds have come from the Muzo mine in Colombia. Emerald is often described as ‘grass green’, but that does not really do justice to these fine stones. In early times they were kept to refresh the eyes and the soul. Try to catch a look at a good one and you can understand why. They are very desirable.
Emeralds are not such a hard stone as sapphires or rubies, rating about 7.5 on the Moh’s hardness scale of 10.
 
Therefore care should be taken with them, especially where emeralds are set in rings when they are at risk of being knocked about.
 
An expensive emerald may be best worn as a pendant or brooch.
 
Cleaning should be undertaken occasionally by a jeweller as all emeralds are oiled to fill in their natural minute cracks.
 
 
Cleaning at home may remove the oil, and damage the appearance of the emerald.

Treatment of emeralds with other fillers or coloured oils should be disclosed by the seller.
 
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