The Strength of Gemstones

Oval.Sapphire.Repair1-2

As previously mentioned a key aspect of a gemstone is that it has sufficient durability. There are three aspects that make up how we assess durability; these are hardness, toughness and stability. It is important for jewellers and gemmologists to know about durability as this can affect how a gem is used, for instance you might not want to put turquoise into an open setting to be worn as a ring as this could easily lead to damage. You must also know the difference between each of the factors as even though emerald is quite hard it is not very tough, so similar care as taken with something like turquoise can also apply to emerald. It is also important in the formation of gem placer deposits, the more durable a gemstone the more likely it is to survive being rolled down a river without breaking up completely (good examples of this are diamonds and jade).

Hardness is the ability of a material to resist abrasion when a pointed fragment of another substance is drawn across it without sufficient pressure to develop cleavage (cleavage will be explained soon) [ref. Gem-A Foundation]. Though testing hardness is not commonly carried out by the average person it is important for many reasons. When cutting a gemstone it is important to know which grinding material to use and it is also a good observational tool for gemmologists identifying stones.

Hardness is usually defined by the Mohs’s Hardness Scale, this scale works by each material being able to scratch the material below it but not the one above it :

Mineral

Hardness

Diamond

10

Corundum (ruby/sapphire)

9

Topaz

8

Quartz

7

Feldspar (Orthoclase)

6

Apatite

5

Fluorite

4

Calcite

3

Gypsum

2

Talc

1

A gemmologist does not need to physically scratch a gem, merely observing a gems surface can help to determine its hardness. For example, you were examining a ring and you could see that it contained a blue stone that had many scratches and worn rounded facet edges you could assume that it might not be sapphire (hardness 9) or  topaz (hardness 8), you could start to presume it might be glass (usually hardness 5-6). The lower hardness of glass would lead to this worn condition as most everyday metals would scratch it and even dust which can contain many particles of quartz (hardness 7) can wear away at glass.

A Blue Glass Ring and a Blue Sapphire Ring A Blue Glass Ring and a Blue Sapphire Ring

Some stones have a different hardness depending on the orientation of the stone; this is due to their crystal structure. It is a bit like trying to cut a piece of wood either along or against the grain. This can be quite important when it comes to cutting a stone like diamond, because diamond is the hardest known gem it can only be cut with other diamonds and because it is slightly less hard in some directions this means that if the soft direction of a diamond is scratched with the hard direction of another, you can start to cut through the stone.

Toughness is the ability of a material to resist the development of fracture (breaking in random directions) or cleavage (breaking along defined planes of atomic bonding in a crystalline material)  through the body of the material[ref. Gem-A Foundation].  Take two green stones:

green-stones Jadeite and Emerald

Jadeite will not often show abundant cracks which can easily break if the stone is knocked, where as an emerald usually shows some kind of fracture or inclusion which usually lowers the toughness of the stone. This is why you rarely find an emerald placer deposit but can find a boulder of jade the size of a car sitting on a river bank.

Lastly, stability is the ability of a mineral to resist physical or chemical alteration due to light, heat or chemical attack [ref. Gem-A Foundation]. A piece of amber is a good example of a gem that is susceptible to heat and will soften at 100oC and yet the process of heating amber in oil is also used to vastly improve the clarity of amber (this produces the “sun-spangled” effect seen in much amber). There are not many gems that are adversely affected by light, but some pink kunzite for example can lose its colour and become white if left under strong light for prolonged periods (such as being left in a well-lit shop window). There are quite a few gems that can be affected by chemical attack, a more commonly known example is pearl which is easily corroded by cleaning agents, pool chlorine etc. Yet even the polish on a peridot may be destroyed by some acids used for jewellery cleaning and repair. A new problem to look out for is lead glass treated ruby and sapphire, although these products are perfectly fine for use as gems (as long as they are disclosed) they can have more adverse effects when treated in the same way as untreated ruby and sapphire when jewellery undergoes cleaning and repair.

Damaged Lead Glass Treated Ruby Damaged Lead Glass Treated Ruby
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