Diamond’s characteristic chemical composition and crystal structure make it a unique member of the mineral kingdom.
Diamond is the only gem made of a single element: It is typically about 99.95 percent carbon. The other 0.05 percent can include one or more trace elements, which are atoms that aren’t part of the diamond’s essential chemistry. Some trace elements can influence its colour or crystal shape.
The way a mineral forms helps determine its identity. Diamond forms under high temperature and pressure conditions that exist only within a specific depth range (about 100 miles) beneath the earth’s surface. Diamond’s crystal structure is isometric, which means the carbon atoms are bonded in essentially the same way in all directions. Another mineral, graphite, also contains only carbon, but its formation process and crystal structure are very different. The result is that graphite is so soft that you can write with it, while diamond is so hard that you can only scratch it with another diamond.
Without any one of these factors, diamond might be just another mineral. Fortunately, though, this special combination of chemical composition, crystal structure, and formation process gives diamonds the qualities that make them extraordinary.
Blue sapphire belongs to the mineral species corundum. It can be a pure blue but ranges from greenish blue to violetish blue. The name “sapphire” can also apply to any corundum that’s not red and doesn’t qualify as ruby, another corundum variety.
The mineral corundum is composed only of aluminium and oxygen, and it requires a growth environment that’s free of silicon. However, silicon is a very common element, making natural corundum relatively uncommon. In its purest state, corundum is actually colourless. Colourless sapphires were once popular diamond imitations, and they’ve staged a comeback as accent stones in recent years.
Blue sapphires come from a variety of exotic sources including Madagascar, Tanzania, Sri Lanka, Myanmar, and Australia.
Red Ruby is the most valuable variety of the corundum mineral species, which also includes sapphire. Rubies can command the highest per-carat price of any coloured stone. This makes ruby one of the most important gems in the coloured stone market.
The strength of ruby’s red depends on how much chromium is present—the more chromium, the stronger the red colour. Chromium can also cause fluorescence, which adds to the intensity of the red colour.
The most renowned rubies, like those from Myanmar, the Himalayas, and northern Vietnam, typically form in marble. They’re found in layers that are distributed irregularly within the surrounding marble. Marble forms as part of the metamorphic (rock-altering) process, when heat and pressure from mountain formation act on existing limestone deposits.
Marble has low iron content, so the rubies that originate in marble (called “marble-hosted” by gemologists) lack iron. Because of this, many have an intense red colour.
Emerald is the green to greenish blue variety of beryl, a mineral species that also includes aquamarine as well as beryls in other colours.
Gem experts differ on the degree of green that makes one stone an emerald and another stone a less-expensive green beryl. Some people tend to give the name emerald to any green beryl coloured by chromium. But to most gemologists, gemological laboratories, and coloured stone dealers, it is more correct to call a stone green beryl when its colour is "too light" for it to be classified as emerald. Even among that group, however, there's a difference of opinion about what's considered "too light."
GIA uses lab-graded comparison stones to determine if the green colour is dark enough and saturated enough to be called emerald.