Fine Coloured Gemstones
Every gemstone carries a story, millions of years in the making, formed under extraordinary pressure and heat, deep within the earth. Here we share what makes each variety remarkable.
Corundum — Al₂O₃ + Iron & Titanium
The stone of wisdom, loyalty and nobility. Sapphire spans every colour of the spectrum, though none more celebrated than the deep velvety blue of Kashmir.
Sapphire's name comes from the Greek sappheiros. Medieval clergy wore blue sapphires to symbolise heaven. The stone has long been associated with faithfulness and sincerity, making it the world's most popular choice for engagement rings, a tradition renewed by Lady Diana's iconic Ceylon sapphire, now worn by the Princess of Wales.
Sapphire is the same mineral as ruby (corundum) but coloured blue by iron and titanium rather than chromium. Any colour of corundum except red is classified as sapphire: yellow, green, pink, orange and the rare padparadscha (the lotus-blossom pink-orange prized above all others).
Ceylon (Sri Lanka), Myanmar (Burma) and Madagascar are the three origins that define the contemporary sapphire market. Ceylon is celebrated for its cornflower blue and exceptional clarity, Burma for deeply saturated royal blues, and Madagascar has emerged over the last two decades as a significant source of fine material that often rivals Ceylon in appearance. Certified top-quality stones from all three origins have seen rapid and sustained appreciation in recent years, driven by growing global demand and tightening supply of fine material.
Kashmir sapphires, mined briefly from 1881 to the early 1900s in a remote Himalayan valley, represent the historical pinnacle of sapphire value. Their hallmark "velvety" blue, caused by minute exsolution platelets scattering light, is inimitable. The deposit has been effectively exhausted for over a century, and certified Kashmir sapphires today appear almost exclusively at major auction houses, where even small, moderately included stones command prices that would be exceptional for flawless material from any other origin.
Heat treatment is standard and accepted by the trade. It improves colour saturation and dissolves silk. Beryllium diffusion, which penetrates deeper than surface diffusion, artificially creates padparadscha-like colours and is considered a significant treatment requiring disclosure. Unheated blue sapphires, particularly from Kashmir and Burma, command meaningful premiums.
Beryl — Be₃Al₂Si₆O₁₈ + Chromium & Vanadium
The stone of Venus. Lush, verdant, and uniquely imperfect. A truly flawless emerald is rarer than a flawless diamond.
Emeralds were mined in Egypt as early as 1500 BC. Cleopatra famously adored them. Spanish conquistadors looted thousands of emeralds from the Aztec and Inca empires, introducing Colombian stones to European courts in the 16th century and permanently shifting global trade. The Mughal emperors of India were among the greatest emerald collectors: the 217.80-carat Mughal Emerald, engraved with prayers, sold at Christie's for $2.2 million.
Colombia remains the world's benchmark for emerald quality. The Muzo, Chivor and Coscuez mines produce stones with the pure, slightly warm green that defines the finest emeralds, a colour that no other origin has convincingly replicated. Colombian origin, particularly Muzo, commands a significant premium at auction.
Zambia has established itself as the most important alternative source, and its reputation has grown substantially over the past decade. The Kagem mine in the Copperbelt (one of the world's largest emerald operations) produces stones with a distinctly cool, slightly bluish-green colour and often exceptional clarity for the species. Top-quality Zambian emeralds now achieve prices that rival mid-range Colombian material, and the origin is fully accepted by all major auction houses. For buyers who prioritise clarity and transparency over the warm Colombian green, Zambia is a genuinely compelling choice.
Emeralds are unique in that virtually all natural stones contain inclusions, known as jardin (garden in French). Surface-reaching fractures are routinely filled with cedar oil, resins or polymers to improve transparency. The GIA and GIA-trained labs grade the degree of enhancement from "None" to "Moderate".
Corundum — Al₂O₃ + Chromium
The king of gemstones, worn by warriors, coveted by kings, and among the rarest minerals on earth.
Rubies have been prized for over 2,500 years. Sanskrit texts called them ratnaraj, king of precious stones. Burmese warriors believed they made them invincible in battle. For centuries, European royalty reserved the finest rubies for crowns and regalia, confusing them with spinels. The "Black Prince's Ruby" in the British Imperial State Crown is in fact a spinel.
The name derives from the Latin ruber, meaning red. Red corundum with a chromium content sufficient to produce a strong red is defined as ruby; below that threshold it becomes pink sapphire, a distinction that significantly affects value.
Origin profoundly affects a ruby's value. The most celebrated rubies come from the Mogok Valley in Myanmar, a remote highland deposit that has been mined for over 800 years. Mogok rubies are defined by their pigeon-blood red: a pure, slightly fluorescent red caused by trace chromium that glows under both natural and UV light. Fine unheated Mogok rubies are among the rarest objects in the gemstone world; significant stones surface at major auction houses only a handful of times per year.
Mozambique emerged as a major source in the early 2000s following the discovery of the Montepuez deposit, currently the world's largest producing ruby mine. Mozambican rubies offer exceptional colour saturation, and the finest stones are now accepted alongside Burmese material at the highest levels of the market. For buyers seeking outstanding colour without the extreme scarcity premium of Burma, Mozambique represents the most compelling alternative available today.
Heat treatment is universal in the ruby trade and fully accepted. Heating at temperatures between 1,600 and 1,800°C dissolves silk inclusions and improves colour. The vast majority of rubies on the market are heated. Unheated rubies of fine quality are exceptionally rare and command a 30–100% premium over equivalent heated stones, depending on origin and certification.
MgAl₂O₄ — Magnesium Aluminium Oxide
The great impostor, the collector's gem. Known for vivid reds and electric pinks, spinel also appears in blue, violet, lavender and black, and is almost always untreated.
For centuries, red spinel was sold as ruby. The most famous example is the 170-carat "Black Prince's Ruby" set into the British Imperial State Crown, which is in fact a spinel. The Timur Ruby, another crown jewel, is also a spinel. The gemological confusion only ended in the 18th century when mineralogists established the two as distinct species. Today, this misidentification has given spinel a unique origin story that sophisticated collectors appreciate.
The Mahenge plateau in Tanzania produces neon-pink spinels of extraordinary electric colour, the most vivid of any gemstone. Mogok, Myanmar, is legendary for red and pink spinels alongside its rubies. The Pamir mountains of Tajikistan have produced large, historically important red spinels for centuries. Vietnam, Sri Lanka and Afghanistan round out the major sources, with Sri Lanka in particular producing fine blues, violets and lavenders.
Spinel is almost always untreated. It simply doesn't respond to heating the way corundum does. This makes it exceptionally rare in the coloured stone world: a fine gem that needs no enhancement. Combined with outstanding hardness (8 on the Mohs scale), excellent brilliance, and a wide colour range including vivid reds, electric pinks, blues and lavenders, spinel represents outstanding value relative to ruby and sapphire of comparable quality.
Grossular Garnet — Ca₃Al₂Si₃O₁₂ + Vanadium & Chromium
Africa's answer to the emerald. Brighter, tougher, and almost always untreated.
Tsavorite is a young gem in geological terms, discovered in 1967 by geologist Campbell Bridges in the Tsavo region of Kenya, near the Tanzania border. Bridges spent years navigating hostile terrain, wildlife and political obstacles before bringing the stone to Tiffany & Co., who introduced it to the luxury market in 1974. Campbell Bridges was tragically killed in 2009 near his mine, but his son Bruce continues to mine the deposit today.
Tsavorite is found only in a narrow geological belt running through Kenya, Tanzania and Madagascar, along an ancient metamorphic terrain called the Mozambique orogenic belt. The finest tsavorites come from the Merelani hills of Tanzania (shared with tanzanite) and from Bridges' original discovery area around Tsavo in Kenya. Madagascar has recently produced some exceptional large stones.
Tsavorite owes its vivid green to chromium and vanadium, the same elements that colour emerald. Unlike emerald, tsavorite is exceptionally clean, has higher refractive index (resulting in greater brilliance), and is virtually always untreated. Stones above 3 carats are rare; above 5 carats are considered museum quality. The colour ranges from mint green through vivid medium green to deeply saturated chrome green.
Zoisite — Ca₂Al₃Si₃O₁₂(OH) + Vanadium
Found in a single square kilometre of earth, in one country, with reserves expected to be exhausted within a generation.
Tanzanite was discovered in 1967 by a Masai tribesman, Ali Juuyawatu, near Arusha in Tanzania, following lightning strikes that started a brushfire and revealed the blue stones in the scorched earth. Tiffany & Co. named it after its country of origin and began marketing it the same year. It has since become one of the most coveted gems in the world, found only in the Merelani Hills at the foot of Mount Kilimanjaro.
Tanzanite is found only in Tanzania, a geographic exclusivity matched by almost no other gem. The entire world supply comes from a mining area approximately 7 km by 2 km near Merelani. Geological surveys suggest reserves will be exhausted within 20–30 years, which has driven increasing interest from investors and collectors. The Tanzanian government has tightened export regulations and developed Block C as a government-controlled mining zone.
In its natural state, tanzanite is strongly trichroic, showing blue, violet and burgundy in different crystal directions simultaneously. Heat treatment at approximately 600°C removes the brownish-red pleochroism, leaving the blue-violet colour the market prizes. Nearly all commercial tanzanite is heated, and this treatment is fully stable, permanent and widely accepted. The finest stones achieve a vivid violetish-blue. GRS grades this as "AAA" or "D-Block" quality.
Know Your Stone
Gemstone treatments are a normal part of the trade. What matters is full disclosure, certification, and understanding how treatment affects value.
Applied to ruby, sapphire and tanzanite. Temperatures of 600–1,800°C improve colour and clarity by dissolving silk inclusions or removing undesirable pleochroism. Fully accepted by the trade and all major laboratories. A well-heated stone shows no structural damage.
Rubies and sapphires found in nature with ideal colour and no heat enhancement are extraordinarily rare. Unheated stones of fine quality command a 30–100%+ premium. All major labs (GRS, Gübelin, SSEF, GIA) test for heat evidence and note "No indications of heating" on the certificate. This is the most valuable status.
Emeralds are almost universally oiled or resin-filled to improve transparency. Labs grade the extent from None to Significant. We only stock emeralds with None, Minor, or Insignificant enhancement. Significant filling is avoided.
A deeper form of lattice diffusion that can transform pale sapphires into vivid yellows and padparadscha-like colours. This is a significant treatment that must be disclosed and cannot be detected by standard visual inspection. Only advanced laboratory testing (EDXRF) can identify it. We do not stock beryllium-treated stones.
Glass or lead-glass filling of surface-reaching fractures in rubies dramatically improves apparent clarity but significantly degrades the stone's stability. Such fillings can deteriorate over time, react to cleaning solvents, and render the stone difficult to set. We do not stock fracture-filled rubies or sapphires.
Spinel and tsavorite are almost always untreated, not because they require enhancement, but because treatments simply don't improve them. This is a significant advantage for the collector: fine, untreated stones with certifiable natural colour are precisely what the most sophisticated buyers seek.
Independent Verification
A gemstone certificate is issued by an independent laboratory with no financial interest in the sale. It is the only reliable confirmation of origin, treatment status and quality. We work with the world's most respected labs.
The benchmark for ruby and sapphire origin determination. GRS pioneered the trade-quality descriptors "Pigeon Blood" and "Royal Blue" that now define the market. Widely regarded as the most important certification for coloured stones at auction.
Switzerland's most prestigious gemmological institution, operating since 1923. Renowned for meticulous origin determinations and Provenance Proof blockchain inscription. Particularly respected for emerald and Kashmir sapphire certifications.
One of the "Swiss trio" alongside Gübelin. SSEF certificates carry equal weight at major auction houses. Known for rigorous scientific methodology and comprehensive gemological reports, particularly for sapphire and pearl.
The world's most recognised gemmological laboratory. While best known for diamond grading, GIA's coloured stone reports are widely accepted and provide excellent market liquidity. Particularly common in the US market.
Antwerp's own laboratory, based in the heart of the diamond district, minutes from our showroom. Widely trusted in the Benelux market. The preferred certification for tanzanite among European dealers and a cost-effective option for stones destined for the European market.
Depending on origin and intended market, stones may also be certified by IGI or reputable local laboratories, including Sri Lankan institutions close to the source. We assess each certificate on its merits and are happy to discuss the issuing lab for any specific stone.
Browse our current inventory of certified coloured gemstones, or tell us exactly what you're looking for and we'll source it.
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