Gemstone value is determined by six quality factors. The first three, colour, clarity, and cut, are covered in part one of this guide. This article covers the remaining three: carat weight, geographic origin, and treatments. These are the factors that most often surprise buyers, because they operate beneath the surface of what you can see with the naked eye.
4. Carat Weight
A carat is a unit of mass equal to 0.2 grams, standardised internationally since 1907. Points are hundredths of a carat, so a 0.75ct stone is sometimes called “75 points.” The name derives from the carob seed, historically used as a counterweight in trade scales across the Mediterranean and Middle East.
Weight and Value Are Not Linear
Larger gemstones are rarer than smaller ones, and price per carat rises non-linearly with size. A single 5-carat sapphire of a given quality will be worth considerably more than five 1-carat sapphires of the same quality combined, because fine material in larger sizes is substantially rarer. The price-per-carat figure in a valuation is not a flat rate; it is a function of size.
Weight Versus Physical Size
Different gem species have different densities, so a 1-carat stone is a different physical size depending on the species. A 1-carat diamond measures approximately 6.5mm in diameter as a round brilliant. A 1-carat ruby, which is denser, measures approximately 6.0mm in the same shape. This matters practically when replacing or setting stones: carat weight and physical dimensions are both needed.
Cutting Trade-Offs
Coloured gemstones typically retain 20 to 50 percent of rough weight after cutting, depending on the crystal shape and cut style chosen. Cutting decisions made to preserve weight can directly compromise proportions, which is one reason cut quality is worth assessing independently of size.
5. Origin
Geographic origin is not a quality factor in the strict technical sense: a sapphire from Sri Lanka and one from Madagascar can be chemically and optically identical. But origin carries significant weight in the market, and it is worth understanding both why and where that influence is rational.
Why Origin Commands a Premium
Certain origins are associated, historically and often accurately, with specific quality characteristics. Burmese rubies from the Mogok Valley are associated with the fluorescent “pigeon’s blood” red produced by high chromium and low iron content. Colombian emeralds from Muzo and Chivor are associated with a warm, slightly yellowish green widely considered the finest expression of the species. Kashmir sapphires are associated with a velvety cornflower blue produced by fine silk inclusions that scatter light evenly.
These associations developed because the geological conditions of those specific deposits consistently produced exceptional stones. The premium reflects both genuine quality tendencies and the scarcity of material from those sources. Kashmir sapphires have not been mined commercially for decades. The premium for confirmed Kashmir origin is partly a quality premium and partly a provenance premium, not unlike a fine wine appellation.
Origin Verification and its Limits
Origin determination is one of the most technically demanding services a gemological laboratory offers. It relies on trace element chemistry, inclusion analysis, and spectroscopic testing. Results are probabilistic, not certain: laboratories issue statements of “consistent with” or “likely origin” rather than absolute determinations. For major origin premiums, a report from a respected institution is essential.
Respected laboratories for origin work include Gubelin Gem Lab, SSEF Swiss Gemmological Institute, GIA, and Lotus Gemology. For high-value stones where origin is part of the price, a report from at least one of these is standard in the serious collector market.
New Sources and Shifting Premiums
Origin premiums shift as new deposits emerge. Mozambique rubies, largely unknown before significant finds around 2009, are now recognised as a source of fine material. Ethiopian opals changed the opal market considerably when they emerged as a major source in the 2000s. Collectors should treat origin premiums as partly historical and partly subject to revision.
Treatments and Enhancements
A large proportion of the commercial gemstone supply has been treated in some way to improve appearance. Treatment status directly affects value. Understanding what treatments exist, how they are detected, and what they mean for price is essential for any serious buyer.
Heat Treatment
Heat treatment is the most widespread treatment in the coloured stone trade. Sapphires and rubies are routinely heated to dissolve or recrystallise silk inclusions, improve colour saturation, and reduce colour zoning. The effects are considered permanent and stable. Heat treatment is widely accepted and disclosed in the trade; an untreated stone of comparable quality commands a significant premium, sometimes 30 to 50 percent or more for fine material.
Emeralds are routinely oiled or resin-filled to reduce the visibility of surface-reaching fractures. Cedar oil was the traditional treatment; synthetic resins such as Opticon are now common. Minor filling is generally accepted; heavy filling reduces value considerably and must be disclosed.
Fracture Filling and Flux Healing
Fracture filling in rubies is more aggressive than emerald oiling. Lead glass filling was widely used to dramatically improve the apparent quality of heavily included, low-grade ruby rough. The resulting stones can appear eye-clean but may be largely glass by volume. Lead glass-filled rubies are significantly less durable than untreated material, prone to damage from heat and household chemicals, and worth a fraction of the price of comparable untreated stones. Detection requires laboratory testing; visual identification is not reliable.
Flux healing involves heating rubies in a flux medium that allows fractures to partially recrystallise. The resulting features are identifiable under magnification by a trained gemologist.
Irradiation and Diffusion
Irradiation is used to produce or intensify colour in a range of gems. Virtually all commercial blue topaz is irradiated, as natural blue topaz is rare. When stable, irradiation is accepted in the trade for certain species and must be disclosed.
Beryllium diffusion, which emerged as a commercial treatment in the early 2000s, caused significant disruption in the sapphire market. The process involves heating sapphires in a beryllium-rich environment to produce or enhance orange, yellow, and padparadscha colours. It can only be detected by laser ablation testing, not by standard gemological equipment. Beryllium-diffused stones are worth considerably less than untreated material and must be disclosed.
Coating and Composite Stones
Coatings alter surface colour or add optical effects. Coated topaz (“mystic topaz”) and coated quartz are common in commercial jewellery. Coatings are not permanent and significantly reduce value relative to naturally coloured stones.
Composite or assembled stones are constructed from two or more materials bonded together: an opal doublet pairs a thin opal slice with a backing material; a garnet-topped doublet layers red garnet over coloured glass. These are not inherently fraudulent but must be disclosed and are worth far less than equivalent solid natural stones.
Disclosure and Documentation
The professional standard is full disclosure of treatments at point of sale. For any stone of significant value, a laboratory report documenting quality characteristics and treatment status is standard documentation that a reputable seller should provide without hesitation. The absence of documentation is not evidence of no treatment. It is simply the absence of documentation.
How the Six Factors Work Together
No quality factor operates in isolation. A ruby with exceptional colour but heavily included clarity may still command a strong price if the colour premium outweighs the clarity penalty. A Kashmir sapphire with moderate colour may still be worth more than a better-coloured stone from a less prestigious origin. A small, untreated, vivid emerald may be worth more per carat than a larger treated one.
The six factors are a framework for understanding, not a ranking of what is worth owning. A stone that scores less well on origin, or that has been heated, or that is smaller than ideal, can still be a beautiful, meaningful, and well-priced acquisition.
Related Reading
For the first part of this guide, covering colour, clarity, and cut.
For anyone thinking about what happens to a gemstone after it is acquired, the museum’s article on legacy and inherited gems explores what it means to pass a stone on across generations.
About The Gem Museum
The Gem Museum in Singapore is a working gemological practice and educational resource. The team includes accredited gemologists offering valuations, collector consultations, and educational programming. Browse the collection and archive at thegemmuseum.gallery.
