A guide to gemstones
Fine jewellery is set with a wide range of stones — from diamonds to coloured gemstones to pearls. Understanding what each stone is, how it forms, and what affects its quality makes it easier to know what you're looking at when you find a piece you like.
Diamonds
Diamonds form deep within the earth under extreme heat and pressure over billions of years. They are the hardest natural substance — a 10 on the Mohs hardness scale — which makes them exceptionally durable for everyday wear.
Diamonds are graded on four characteristics: cut, colour, clarity, and carat weight. Cut determines how the stone handles light and affects its brilliance. Colour is graded from D (colourless) to Z (yellow tint). Clarity refers to internal inclusions, many of which are invisible to the naked eye. Carat is a measure of weight, not size.
Natural vs lab-grown
Lab-grown diamonds are produced in controlled environments using one of two processes: High Pressure High Temperature (HPHT), which replicates the conditions under which natural diamonds form underground, or Chemical Vapour Deposition (CVD), which grows a diamond crystal from a carbon-rich gas. The result is a stone that is chemically, physically, and optically identical to a natural diamond.
Both are graded using the same standards by the same independent laboratories. The difference is origin, not composition. Natural diamonds tend to hold their resale value better; lab-grown diamonds are available at a significantly lower price point for equivalent grades. Sellers are required to disclose whether a diamond is natural or lab-grown — the two cannot be distinguished without specialist equipment.
Sapphires
Sapphires belong to the corundum family — the same mineral as rubies, coloured blue by traces of iron and titanium. They are the second hardest gemstone after diamonds (9 on the Mohs scale), making them well suited to everyday wear including engagement rings.
The word sapphire is most associated with blue, ranging from pale sky blue to deep cornflower to rich royal blue. However, sapphires occur in almost every colour — pink, yellow, orange, green, and white. A padparadscha sapphire, a rare pink-orange variety, is among the most prized stones in the world.
Colour is the primary quality factor: intensity, saturation, and evenness of tone. Heat treatment to improve colour and clarity is standard practice across the trade and does not diminish a stone's value when disclosed. The finest sapphires have historically come from Kashmir, Burma, and Sri Lanka.
Rubies
Rubies are corundum — chemically identical to sapphires, but coloured red by the presence of chromium. The distinction between a ruby and a pink sapphire is one of degree: how red is red enough to be called a ruby is a matter the trade continues to debate.
The most valued rubies have a vivid, pure red — sometimes described as pigeon's blood — without brown or orange undertones. Origin matters significantly in price: rubies from the Mogok valley in Burma are among the most sought after. Heat treatment is widely used and accepted. Fracture filling — where glass or resin is used to improve the apparent clarity of a stone — is more controversial, must be disclosed, and affects value.
Fine rubies are among the rarest gemstones in the world. At the top of the market, exceptional stones command higher prices per carat than diamonds of equivalent size.
Emeralds
Emeralds are a variety of beryl — the same mineral family as aquamarine — coloured green by chromium and vanadium. They rank 7.5–8 on the Mohs scale, somewhat softer than sapphires and rubies, and more susceptible to chipping if knocked.
Almost all emeralds contain inclusions — internal fractures, crystals, and growth characteristics formed during development. The trade refers to these collectively as the stone's jardin (from the French for garden). Unlike with diamonds, inclusions in emeralds are accepted as part of the stone's character rather than strictly penalised; a completely eye-clean emerald of good colour is exceptionally rare.
Oil treatment — filling surface-reaching fractures with cedar oil or resin to improve apparent clarity — is standard and widely accepted practice, provided the degree of treatment is disclosed. Colombia has historically been the most important source of fine emeralds.
Pearls
Pearls are organic — formed by molluscs rather than geological processes. A natural pearl forms when an irritant enters a mollusc and the animal coats it in layers of nacre (mother of pearl) over time. Natural pearls of significant size are exceptionally rare and correspondingly valuable.
The vast majority of pearls sold today are cultured — formed through the same natural process, but initiated by humans who introduce a nucleus into the mollusc. Cultured pearls are real pearls. Freshwater pearls are cultured in mussels, primarily in China. Saltwater varieties include Akoya pearls (Japan), Tahitian pearls (French Polynesia), and South Sea pearls (Australia, Indonesia, the Philippines) — the largest and most valuable of the cultured varieties.
Quality factors for pearls include lustre (the brightness and depth of the reflection at the surface), surface quality, shape, colour, and size. Round pearls with high lustre and clean surfaces command the highest prices, though baroque (irregular) pearls have their own appeal.
Other gemstones
Fine jewellery is set with a wide range of stones beyond the four traditional precious gems. Some of the most commonly encountered:
Blue-green beryl, the same family as emerald. Pale to deep blue, typically with good clarity. Heat treatment to reduce green tones is common.
Violet quartz, ranging from pale lilac to deep purple. Widely available and durable. Heat treatment can convert it to citrine or prasiolite.
Known for its play of colour — the iridescent flashes that shift with the angle of light. Relatively soft (5.5–6.5 Mohs); requires care and a protective setting. Australian opals are the most prized.
Occurs in a wide range of colours. Blue topaz — often enhanced by irradiation — is the most widely used. Imperial topaz, a golden orange-yellow, is the most valued variety.
One of the widest colour ranges of any gemstone — from deep green (chrome tourmaline) to vivid pink and red (rubellite) to the rare blue-green Paraíba variety, which commands very high prices.
Most associated with deep red, though garnets occur in almost every colour. Tsavorite (vivid green) and spessartite (orange) are particularly sought after.
Precious vs semi-precious
The terms precious and semi-precious have been used in the jewellery trade for centuries. Traditionally, four stones were classified as precious: diamonds, rubies, sapphires, and emeralds. Everything else was semi-precious.
The distinction is increasingly considered outdated. A fine alexandrite, Paraíba tourmaline, or exceptional garnet can command far higher prices than many diamonds. The terms persist in common use but are not a reliable guide to value, and are not used within the professional trade as any kind of quality standard.
Treatments
Most gemstones on the market have been treated in some way to improve their appearance. Treatment is standard practice across the trade, but the nature and extent should always be disclosed.
Applied to sapphires, rubies, aquamarine, and many other stones to improve colour and clarity. Widely accepted; should be disclosed.
Used primarily on emeralds — and some rubies — to fill surface-reaching fractures. Accepted when minor; significant treatment affects value. Should be disclosed along with the degree.
Glass or resin injected into fractures in diamonds or rubies to improve apparent clarity. More controversial than oiling; affects durability and value. Must be disclosed.
Used to alter colour in topaz, diamonds, and other stones. Generally stable and widely accepted; should be disclosed.
A thin layer applied to the surface to alter colour. Less stable than other treatments and can wear over time. Should be disclosed.
When buying any significant stone, it is reasonable to ask whether it has been treated and to what degree. Grading reports from independent laboratories will note any treatments identified.
