Aquamarine Quality Factors
Aquamarine’s colour range is very narrow: It can be blue, very slightly greenish blue, greenish blue, very strongly greenish blue or green-blue. The gem’s most valuable colour is a dark blue to slightly greenish blue with moderately strong intensity. In general, the purer and more intense the blue colour, the more valuable the stone. Most aquamarine is a light greenish blue.
Although some buyers prefer the more greenish natural colour, most of the aquamarine in the market has been heat-treated to give it more of a pure blue.
Fashioned aquamarines often have to be fairly large—generally over 5 carats—to show intense, dark colour. Although small gems are rarely saturated enough to be attractive, stones from some mines in Africa—Nigeria, Madagascar and Mozambique, for example—are known for intense colour in sizes under 5 carats. For this reason, smaller, top-colour stones might sell for more per carat than larger stones of the same colour.
Most faceted aquamarines are eye-clean. Some crystals might contain liquid inclusions, but clarity characteristics are few or absent in most finished gems. Stones with eye-visible inclusions are usually fashioned into cabochons, beads or carvings.
In some beryl crystals, there are enough parallel inclusions—usually long hollow or liquid-filled tubes—to allow cutters to fashion the rough to show a cat’s-eye.
Aquamarines can be cut into almost any shape, but cutters often fashion them as emerald cuts or as round or oval brilliants. The rough is fairly plentiful, so well-cut stones are fairly common. The gemstone’s hardness and transparency make it popular with designers, artists and carvers. Gem sculptors use aquamarine for fantasy cuts and ornamental objects.
The gem is pleochroic, which means it shows different colours in different crystal directions—in the case of aquamarine, they’re near-colourless and strong blue. Fortunately, the blue pleochroic colour corresponds with the cutting orientation that retains the most weight, with the table facet aligned parallel to the length of the crystal.
Aquamarine crystals come in sizes from very small to very large — some even up to 100 lbs. (45 kg). While large stones are readily available, it’s difficult to use them in jewellery, so there’s less demand for them, except as centre stones. As a result, per-carat prices tend to decrease for sizes above 25 carats.
Many very large aquamarine crystals have been discovered. The largest Brazilian aquamarine on record was found in 1910, in Minas Gerais, Brazil. It weighed 244 lbs. (110 kg) and measured 19 in. (48 cm) long and 15 in. (38 cm) in diameter.