Famous Gems
Gemstones are minerals (and a few rocks and organic materials) that are valued for their beauty, rarity and durability. They are usually cut and polished to bring out their colour and sparkle. The four main "precious" gems have been treasured for thousands of years: diamond, ruby, sapphire and emerald. Dozens of other "semi-precious" stones (amethyst, topaz, garnet, opal, turquoise, jade and many more) are also popular. Some individual famous gems have their own names and dramatic histories of theft, war, royal owners and curses, making them objects of legend as well as beauty.
- The "Big Four" precious gemsDiamond, ruby, sapphire, emerald
- Most famous diamondHope Diamond45.5 carats, deep blue
- Largest rough diamondCullinan3,106 carats (621 g), South Africa, 1905
- Most famous rubySunrise Ruby25 carats, sold for $30 million in 2015
- Birthstone tradition12 monthly stonesDifferent gem for each month
- Carat (weight unit)200 milligrams1 carat = 200 mg
The four precious stones
- Diamond: pure carbon, the hardest natural substance. Formed deep in the mantle and brought to the surface by ancient volcanic eruptions. Most prized when colourless, but also valued in pink, blue and yellow.
- Ruby: the red variety of corundum (aluminium oxide). Coloured by traces of chromium. The most valuable rubies have a "pigeon blood" red colour.
- Sapphire: any other colour of corundum, but most famously deep blue. Coloured by traces of iron and titanium.
- Emerald: green variety of beryl. Coloured by traces of chromium and vanadium. Famous for almost always containing tiny inclusions (called the "garden" of the emerald).
The Hope Diamond
One of the most famous gems in the world is the Hope Diamond: a deep blue, 45.5-carat diamond now on display at the Smithsonian in Washington, DC. It was mined in India centuries ago, sold to a French jeweller in the 1660s, owned by King Louis XIV of France, stolen during the French Revolution, eventually cut down and re-sold, and bought by a London banker called Henry Philip Hope in the 1830s.
The diamond changed hands many times over the next century, gaining a reputation for bringing bad luck to its owners (although the "curse" story was largely invented by jewellers and journalists to drive up interest). In 1958 the famous jeweller Harry Winston donated it to the Smithsonian, where it is now one of the most-visited objects in any museum in the world.
The Cullinan Diamond
The largest gem-quality rough diamond ever found is the Cullinan Diamond, discovered in South Africa in 1905. The original stone weighed an astonishing 3,106 carats (621 grams), about the size of a fist. It was bought by the Transvaal government and given to King Edward VII of Britain as a birthday present.
The rough diamond was carefully studied and then cut by Joseph Asscher of Amsterdam in 1908. Asscher reportedly spent weeks studying the stone before making the first cut, and is said to have fainted when the diamond split exactly as planned. The Cullinan was cut into 9 major stones and many smaller ones. The two biggest pieces (Cullinan I "The Great Star of Africa" at 530 carats, and Cullinan II "The Lesser Star of Africa" at 317 carats) are part of the British Crown Jewels and are on permanent display at the Tower of London.
How gems form
Most gemstones form deep underground over very long periods of time. The exact process depends on the gem.
- Diamonds: form at depths of 150+ km in the Earth's mantle, where temperatures and pressures are extreme. They are carried up to the surface by rare deep volcanic eruptions called kimberlite pipes.
- Rubies and sapphires: form in metamorphic rocks where aluminium-rich rocks have been cooked and squeezed for millions of years.
- Emeralds: form in special hydrothermal veins where chromium-rich rocks meet beryllium-rich rocks.
- Opal: forms when silica-rich water seeps into cracks and slowly dries out, leaving microscopic spheres of silica that diffract light into the famous opal "fire".
- Pearls: not really a gemstone at all, but formed inside the shells of certain oysters and mussels around an irritant.
- Amber: not a mineral at all, but fossilised tree resin.
What gives gems their colour?
Most gemstones are basically the same chemical compound as some common mineral, but with traces of certain metallic elements that produce vivid colours.
- Ruby and red garnet: chromium.
- Sapphire: iron and titanium.
- Emerald: chromium or vanadium.
- Amethyst: iron.
- Turquoise: copper.
- Yellow topaz: iron.
- Pink and blue diamonds: structural defects rather than impurities.
Deeper dive: the human cost of gemstones
The diamond and gemstone industry has a troubled history. For much of the 20th century, "blood diamonds" (also called conflict diamonds) were mined in war zones in central and west Africa, with the profits used to fund civil wars in countries like Sierra Leone, Angola and the Democratic Republic of Congo. Hundreds of thousands of people died in these wars.
In 2003, the international Kimberley Process was set up to certify that diamonds had not been mined in conflict zones. While not perfect, it has dramatically reduced the trade in conflict diamonds. Today, most diamonds sold in major markets carry a certificate of origin.
Other gemstone supply chains can still be problematic. Many gemstones are mined in small unregulated operations in poor countries, where miners (often including children) work in dangerous conditions for very little money. Some gemstones (especially certain emeralds and rubies) have been linked to environmental damage and labour abuses.
One way around this is lab-grown gems. Since the 1980s, scientists have been able to grow real diamonds, rubies and sapphires in laboratories from the same raw materials and the same crystal structures as natural stones. Lab-grown diamonds are physically and chemically identical to mined diamonds, just made in weeks rather than billions of years. They typically cost 30 to 80% less than mined gems and have a much smaller environmental and human impact. As of 2025, lab-grown gemstones are taking a growing share of the global market.