Basics
To use the definition on the previous page, a gem is a material used in decoration or ornament to which we associate the criteria of beauty, rarity, durability and market value.
Note that gemology applies to a gemstone as well as an ornamental stone and even certain organic materials such as amber, pearls and coral. Whatever its quality, an agate cannot be considered a gem. And yet, the gemologist must also know it.
In France, the name “precious stone” was reserved for the diamond, ruby, sapphire and emerald quartet. The rest of the stones constituting either the fine stones or the ornamental stones. Since2002 , these appellations no longer exist for precious stones and semi-precious stones. We are talking about gemstones. This is a great thing because a good quality tanzanite is worth much more than an opaque ruby. This tanzanite will be called gem, but not the stony ruby.
Each gem can be natural, with or without treatment. There are also synthetic stones, imitations and composite stones.
Take the example of a sapphire. The majority of sapphires are heated as this enhances its color if it was too pale, and also helps to make it a bit more transparent. It is indeed a natural stone, but it has received a heat treatment. This is accepted by the profession because it is a current and stable treatment over time. It will not be mentioned on purchase. Yet an identical but unheated sapphire would have had a significantly higher market value.
Take the case of a sapphire with cracks. Another treatment is to heat it under vacuum in the presence of cobalt (blue) glass. This glass will penetrate the cracks and soften them tremendously. This is a treatment as well, but this must be mentioned when purchasing as it drastically lowers the price of the stone compared to a similar but untreated stone! This is a common treatment with ruby. Most of the raspberry-pink rubies on the market are treated with lead glass.
Let us now take the case of the emerald. If the emerald is treated with oils, the treatment is tolerated. If it’s with resins, it isn’t. Each stone is different.
A synthesis is an identical reproduction of an existing gem. For example, we can produce corundum by the Verneuil method which consists of fusing alumina powder. I advise you to watch the film ” Burmese pigeon’s blood ruby ” by Patrick Voillot for free on Youtube, because at the very end of the film, a Verneuil ruby production plant shows the manufacturing process.
An imitation is a material that is offered in place of another. For example, zirconium oxide, which is a synthetic material not to be confused with zircon, is a convincing imitation of diamond. Spinel is an excellent imitation (or substitution) of corundum. Kyanite is also an excellent imitation of sapphire.
Composite stones are stones assembled artificially. For garnet-glass doublets, a glass bead is melted on a small wafer of garnet. The garnet plate is very thin, so the color of the doublet will be given by the color of the glass. Garnet containing natural inclusions, we can confuse the doublet with a natural stone.
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