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Sorting and Grading Coloured Gemstones

Gemfields’ proprietary grading system is key to the modernisation of the coloured gemstone industry and the success of auctions. Buyers can now have confidence in their purchases and design increasingly ambitious colourful jewellery creations.

Sorting and Grading Emeralds

Before Gemfields acquired Kagem in 2008, in partnership with the Zambian government, sorting and grading systems for emeralds were inconsistent. Buyers bought bags of mixed-quality gemstones.

Gemfields’ proprietary grading system sorts gems into more than 200 grades, giving buyers more confidence in their purchase.

Lots of similar size, colour and clarity are sold for the highest price at invitation-only auctions, with all proceeds fully repatriated to Zambia.

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The Process
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Sorting

Every day, securely locked boxes containing recovered Zambian emeralds arrive at Kagem’s sorting house. Exceptional emeralds go straight to the sorting room.

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MANUAL COBBLING

The majority of emeralds are carefully cobbed by hand – slowly chipping away at the host rock to reveal the gemstone inside.

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TUMBLING

The higher quality emeralds move on to the sorting room, while the remainder are placed in huge, slow-moving tumblers that remove any rough edges.

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SEPARATION

A vibrating belt separates the gemstones by size, before a final stage of cobbing by small hand tools recovers every piece of emerald as delicately as possible, using a state-of-the-art washing system. High-pressure air removes any remaining rock, revealing a breathtaking array of emeralds.

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Emerald Grading

Specialists lightly clean the gemstones and sort them according to colour, size, shape and clarity.

Sorting and Grading Rubies

When grading a ruby, colour is crucial. The primary hue must be red, but secondary hues can be pink, orange and purple.

Rubies from different origins often have distinctive hues, but Mozambican rubies span all known colour ranges, including the rarest pure fluorescent reds. They also have the most sought-after characteristics that make a ruby desirable: fluorescence, colour and clarity perfection.

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The Process
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Sorting using State-of-the-art equipment

In 2019, Montepuez Ruby Mining introduced the most technologically advanced coloured gemstone sort house in the world. Equipment includes optical sorting machines, which use ultraviolet light to identify individual gemstones, and then jets of air to push them free.

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Sorting by hand

The remaining gravel is sorted by hand in sealed, windowed boxes, with specially trained sorters to recover the last of the rubies.

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Grading

Gemstones are then graded for colour, size, shape and clarity.

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Consistency Check

Rubies are checked against the Gemfields reference sets to ensure consistency. This is critical to maintaining buyer confidence and key to the success of auctions and the modernisation of the coloured gemstone industry.

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Auction sales via our Pioneering auction platform

Rubies are separated into lots for sale through a closed-bid auction. The proceeds are fully repatriated to Mozambique and all royalties due to the government paid on the full sales price.

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