Rough Sorting for Gem Cutting

Determining the cost of a piece of rough can be a daunting task. This is compounded by the complexity of the unique properties exhibited by each mineral type and the value of a piece of rough mineral can change dramatically depending on these properties.

One of the most important factors which influence the value of a piece of mineral is the colour. Mineral experts estimate that the colour component of a mineral can account for as high as 50% to 70% of its value. This is understandable as the colour intensity is the first thing that draws attention to the mineral.

The body colour of a piece of mineral is considered in terms of the hue, tone and saturation. The hue is the base colour that first stands out when the mineral is viewed and the tone describes the degree of lightness or darkness of the colour. A mineral specimen with a dull colour will not stand out, but a specimen such as rhodochrosite with a striking colour commands a higher price.

Colour saturation describes the intensity and depth of the colour. Minerals with washed out or irregular colours are much less valued than those with a higher degree of colour saturation. However, the level of saturation can influence the hue of the colour. Therefore, the desirable level of saturation is moderate to high, which is consistent throughout the body of the mineral.

Clarity and transparency
Most natural mineral specimens have inclusions which impact on their level of clarity and transparency. The more transparent a mineral specimen is, the more valuable. Minerals such as emeralds are naturally formed with inclusions and are commonly found with them. Emeralds with a high level of clarity are rarely found. Such emeralds are therefore highly valued. On the other hand, aquamarines are commonly found with a high level of clarity. Therefore, aquamarines with visible inclusions often command lower prices.

Fractures and cleavages
Damage to mineral specimens cause fractures that may sometimes be extensive. This can present a problem for gem cutters with gemstones, especially if the specimens are to be cut and polished. Some specimens can be repaired depending on the degree of damage, but a fractured mineral would usually command a lower price than an undamaged one.

Mineral size
Large and well-formed minerals command a higher price. Natural crystals tend to have minor deformities and the larger they are, the more likely they will be deformed. Therefore, large crystals which are well-formed are unique and will attract a higher cost.

In general, gem minerals are valued according to their carat weight. One carat is equivalent to about 0.2 grams. Due to the difference in the densities of various gem varieties, one carat of different gemstones will vary in size. Therefore, one carat of sapphire is smaller than one carat of emerald. Also, gemstones such as rubies which are rarely found in large carat sizes command higher prices than aquamarines which are commonly found in very large carat sizes.

Rarity and demand
Rare minerals tend to be valued higher than the commonly occurring ones, even when the latter are nicely formed and well coloured. For this reason, rare minerals such as the benitoite are highly priced due to their rarity.

The origin of a mineral specimen plays a role in its valuation. Specimens obtained from certain classic locations are reputed for their quality and therefore command a higher premium than those found in other locations.

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