Often, terms like “gemstone” and “precious stone” are used interchangeably, but they’re not quite the same. Any colored, mineral-based crystal, as well as some other organic-based materials, used in the manufacture of jewelry can be called a gemstone, but there are also different groups within the broader class, largely based on rarity rather than quality.
In strict terms, there are only 4 precious stones – diamond, sapphire, emerald and ruby – and all other gemstones are usually referred to as semi-precious. Although that might sound reasonable and straightforward, the issue is clouded by the fact that a good semi-precious stone will often carry a higher price than an average precious stone. The two groups have their origins in classifications determined by the ancient Greeks, and it is testament to the advanced knowledge they had in such matters that the same rules remain in place today.
As we’ve said, though, only diamonds, sapphires, emeralds and rubies actually qualify to be called precious stones.
Diamond
Although technically an element, and not a mineral, carbon crystallines – of which a diamond is one – are still classed as minerals. Despite it’s position at the very top of the precious stone tree, a diamond is something of a curiosity in pure geological terms. There are two pure carbonite minerals, diamond and graphite, of which diamond is the hardest of all known natural materials and graphite one of the softest. What makes carbon in the earth’s structure form a diamond and not graphite is a combination of heat, pressure and the way the carbon elements bond. As a result, although chemically almost identical, the atomic structure determines all the qualities that make one or the other.
Diamonds are the hardest natural substance known to man, and cannot be cut or otherwise scratched or marked by anything other than a second diamond. Flaws, or inclusions, are often not seen by the naked eye, and even with a jeweler’s loupe still require a trained eye to identify. In the case of structural flaws, even the most skilled diamontiere risks a diamond splitting or even shattering, if care isn’t taken to work around the inclusion. For this reason, flawless, colorless cut diamonds are highly prized and priced to match the effort that has gone into the finished cut.
The 4Cs of of classification – Color, Clarity, Cut and Carat – determine the nominative quality of the stone itself, and each can affect the value considerably, despite the subtle differences between the gradings often being impossible to see with the naked and/or untrained eye. Despite the misconception that the 4Cs only apply to diamonds, they actually apply to all precious stones.
The romantic associations with diamonds means demand will always be high, and the skill of the jeweler will ensure that this most precious of stones will always be at the top of anyone’s list.
Sapphire
An example of the mineral corundum, sapphires are commonly blue in color, but can come in a wide range of hues. These “fancy” sapphires are quite common and are found in green, purple orange and yellow, each color caused the presence of one or more of iron, titanium, chromium, copper and magnesium. Sapphires have also been successfully produced synthetically, and these laboratory-created stones are almost visibly indistinguishable from the natural stones. Synthetic sapphires are used in a wide-range of industrial applications, where the hardness of the stones – 9 on the Mohs Scale compared to a diamond’s hardness rating of 10 – applies well to products such as shatter-resistant glass and as a component in body armor.
Despite the relative ease with which synthetic sapphires can be produced, natural sapphires are still the most desired form of this beautiful gem. Natural sapphires have been heat treated since Roman times, to improve the depth of color, and it is estimated that up to 95% of natural sapphires are subjected to this process. This makes non-heat treated stones extremely desirable and highly marketable, with sapphires mined in Sri Lanka and Kashmir being particularly sought after.
Unusually for precious stones that are used in jewelry manufacture, flaws or inclusions are often exploited, rather than avoided. An example is the star inclusion, caused by the presence of the mineral rutile which produces a star-effect in the sapphire. The star, if present, is usually positioned in the top center of the stone, making cabochon stones (polished and shaped, but rarely faceted) common with star sapphires.
The deep blue of a high quality sapphire is breathtaking, and they pair superbly with diamonds thanks to the contrast between the two colors. Such pieces are assured of admiring looks and comments, and the sapphire needn’t have a star to be a showstopper.
Emerald
An emerald is formed as a result of the mineral beryl being colored by the presence of chromium. Although significantly softer than diamonds, emeralds still rank fairly high on the Mohs Scale of Hardness, at around the 8 mark. The difference is that, unlike diamonds or sapphires, emeralds are often heavily included, and so are prone to splitting or breaking during cutting and polishing.
Where sapphires are heat treated to enhance both color and transparency, transparency in emeralds is achieved by the addition of oils that have a similar refractive index to the natural beryl. This helps to strengthen the stone and also to fill any surface cracks that may be present. Like all precious stones, non-treated emeralds carry a much higher value than a treated stone of the same visible quality.
One unusual aspect of emerald grading is that it is done by eye, and without the use of the common 10x loupe as used when grading other stones such as diamonds. If no inclusions can be seen with the naked eye, then an emerald can be considered flawless. Because of this, in the US a treated emerald must be declared as such by law, so as to maintain the integrity of the market when dealing emeralds. The ability to determining trace element differences in the composition of emeralds makes it possible to identify the location of the mine that produced it, as all have their own unique fingerprint.
Ruby
Almost everything you need to know about rubies can be found by reading the above information about sapphires as, in fact, a ruby is no more or no less than a red sapphire. The name comes from the latin ruber, meaning red, and it gets its color from the presence of chromium. Although sapphires also often contain chromium, only in rubies is there no other trace element present, creating a deep red stone without any influence that may cause other colors to be created.
As with any precious stone, the richer the color, the higher the value. Because of this, pink rubies have had their own classification in the US for some time, although this practice has not been adopted elsewhere, and a sapphire of any pink or red hue will be classified as a ruby. Similarly to sapphires, rubies are also heat-treated to improve the color, but with the more focused aim of removing any purple or blue patches to leave a more consistent red color across the entire stone.
Myanmar (previously Burma) has long since been considered to produce the finest rubies, the largest of which was a stone of 8,500 carats, and weighing 4 pounds. The Liberty Bell Ruby, as it came to be known due to being sculpted into a scale replica of the actual bell of the same name was stolen in 2011 and, although several arrests were made, the chances of ever finding the ruby are thought to be slim.
An industrial use of rubies is in the manufacture of lasers. The red light that we see in laser beams is red for the very reason that it is produced by using a ruby to produce the focused beam of light on a very specific wavelength. It is, though, the beauty of the natural stone for which the ruby is universally adored.
Spectacular in any setting, and a superb compliment to almost any other gemstone, with its association with the heart and love, it’s no wonder that rubies are one of the most popular of all stones used in jewelry.