A collector’s field guide to the world’s great gems, and the questions to ask before you buy one. A gift for readers of The Friday Carat.
The difference between a jewel you love for a season and one you hand down usually comes down to what you knew at the counter. Where the stone came from. What was done to it. Which lab looked at it, and what the report actually says.
This guide covers the ten stones we write about most at The Carat Post, from the diamond everyone knows to the neon tourmaline most people have never heard of. For each one you’ll find what makes it special, the facts that drive its value, and the single question worth asking before you buy.
At the end: a checklist to take to any jeweler, and a quick care chart so the stone you choose stays beautiful.
01. Diamond
The benchmark every other stone is measured against
Pure carbon, crystallized deep in the earth, and the hardest natural material known. A diamond’s beauty is mostly human work: the cut decides how much light comes back to your eye, which is why two stones of the same weight can look worlds apart.
Today the first question is no longer only the 4Cs. Lab-grown diamonds are chemically and optically the same as mined ones and cost a fraction of the price, so a stone’s origin, natural or laboratory-grown, now matters as much as its grade.
| Mineral | Carbon |
| Hardness | 10 (Mohs) |
| Value drivers | Cut, color, clarity, carat |
Ask: “Is there a grading report from GIA or IGI, and does it state natural or laboratory-grown?”
02. Ruby
The king of gems
Ruby is red corundum, colored by chromium. The most celebrated stones come from the Mogok valley in Myanmar, whose finest glowing red earned the trade name “pigeon’s blood.” Mozambique is now one of the most important sources of fine rubies.
Most rubies are heated to improve color and clarity, which is accepted when disclosed. An unheated stone of top color is rare and priced accordingly. Be wary of lead-glass-filled rubies: heavily treated material worth far less and more fragile.
| Mineral | Corundum |
| Hardness | 9 |
| Value drivers | Color, origin, heat or no heat |
Ask: “Does a lab report state the origin and whether the stone has been heated or filled?”
03. Blue Sapphire
Royal blue, cornflower and the rest
Sapphire is corundum like ruby, but blue from traces of iron and titanium. Three origins carry the most prestige: Kashmir, whose velvety stones came largely from a short burst of mining in the 1880s; Burma, known for deep royal blues; and Sri Lanka, historically Ceylon, prized for brighter, livelier tones.
As with ruby, heating is common and accepted. A fine unheated sapphire with a respected origin commands a significant premium, which is exactly why a lab report matters.
| Mineral | Corundum |
| Hardness | 9 |
| Value drivers | Color, origin, treatment |
Ask: “Is the origin stated by an independent lab such as SSEF, Gübelin, GRS, AGL or GIA?”
04. Padparadscha Sapphire
The color of a lotus at sunset
A delicate blend of pink and orange, named from the Sinhalese word for the lotus blossom. Sri Lanka is its classic home, with fine stones also found in Madagascar. True padparadscha is one of the rarest colors of sapphire, and collectors guard the definition closely.
In the early 2000s, large numbers of orange-pink sapphires turned out to have been treated with beryllium diffusion to create the color. That is why a report confirming the color is natural is essential here.
| Mineral | Corundum |
| Hardness | 9 |
| Value drivers | Balance of pink and orange |
Ask: “Does the report call it padparadscha, and rule out beryllium diffusion?”
05. Emerald
A garden inside a stone
Emerald is green beryl, colored by chromium and vanadium. Colombia, and especially the Muzo mine, has produced the most revered stones for centuries; Zambia and Brazil are major modern sources. Nearly every emerald has inclusions, lovingly called its jardin, or garden.
Filling surface-reaching fissures with oil or resin is standard practice. What matters is how much. Emeralds are also more brittle than their hardness suggests, so they deserve protective settings and gentle cleaning.
| Mineral | Beryl |
| Hardness | 7.5–8 |
| Value drivers | Color, clarity enhancement, origin |
Ask: “What degree of clarity enhancement does the report show: none, minor, moderate or significant?”
06. Spinel
History’s great impostor, now a star
For centuries red spinel was mistaken for ruby. The famous “Black Prince’s Ruby” set in Britain’s Imperial State Crown is in fact a spinel. Today it is loved on its own terms, in reds, hot pinks, lavenders and a rare cobalt blue.
Spinel is usually untreated and costs far less than a ruby of similar color, which makes it one of the collector’s smartest buys. Mogok in Myanmar, Tanzania and Vietnam produce some of the finest material.
| Mineral | Spinel |
| Hardness | 8 |
| Value drivers | Saturation, size, color rarity |
Ask: “Is it natural spinel? Synthetic spinel has been made for over a century.”
07. Paraíba Tourmaline
Neon from the deep
Discovered in the Brazilian state of Paraíba in the late 1980s, this copper-bearing tourmaline glows an electric blue-green unlike anything else in nature. Similar copper-bearing stones were later found in Nigeria and Mozambique.
Most labs now use the name “Paraíba” for the copper-bearing variety wherever it was mined, and state the country separately. Brazilian stones remain the most prized. Sizes over a few carats with intense color are exceptionally rare.
| Mineral | Tourmaline |
| Hardness | 7–7.5 |
| Value drivers | Neon intensity, origin, size |
Ask: “Does the report confirm copper, and which country the stone comes from?”
08. Alexandrite
Emerald by day, ruby by night
A rare chrysoberyl that changes color: green to bluish green in daylight, red to purplish red under warm incandescent light. It was found in Russia’s Ural Mountains in the 1830s and named for the future Tsar Alexander II. Brazil, Sri Lanka, India and East Africa produce stones today.
The strength of the color change is everything. Many “alexandrites” sold to tourists are synthetic, or color-change sapphire and garnet, so buy with a report.
| Mineral | Chrysoberyl |
| Hardness | 8.5 |
| Value drivers | Strength of color change |
Ask: “Can I see it under daylight and incandescent light, and is it natural?”
09. Tanzanite
A one-mine stone
Discovered in 1967 in the Merelani Hills of northern Tanzania, still its only known commercial source, and introduced to the world by Tiffany & Co. the following year. Its blue-violet color shifts as you turn it, and the finest stones show a deep, saturated violet-blue.
Almost all tanzanite is heated to reach that color, which is standard. It is softer than sapphire, so it suits earrings and pendants better than a ring worn every day.
| Mineral | Zoisite |
| Hardness | 6–7 |
| Value drivers | Depth of color, size |
Ask: “Is the setting protective enough for how often I’ll wear it?”
10. Pearl
The only gem made by a living creature
Pearls are organic gems. Natural pearls form without human help and are extraordinarily rare; almost all pearls sold today are cultured, from Akoya, South Sea, Tahitian and freshwater farms. Both can be beautiful, but the price gap is enormous.
Judge luster first: the sharpness of reflections on the surface. Then surface cleanliness, shape, size and matching. Pearls are soft, so they should be the last thing you put on and the first you take off.
| Material | Nacre |
| Hardness | 2.5–4.5 |
| Value drivers | Luster, surface, shape, size |
Ask: “Natural or cultured? A natural pearl should come with a lab report.”
Six questions to ask before you buy any gem
- Is there an independent lab report? For important stones, look for GIA, SSEF, Gübelin, GRS or AGL. A report from the seller’s own lab is not the same thing.
- Is it natural, synthetic or lab-grown? Lab-created rubies, sapphires, spinels, emeralds and diamonds are real gems, but priced very differently from natural ones.
- Has it been treated, and how? Heating, oiling and filling are common. Disclosure is required by trade guidelines; the type and degree change the value.
- Where was it mined? For rubies, sapphires, emeralds and Paraíba tourmaline, origin can move the price dramatically. Only a lab can support the claim.
- Does the stone match the report? Check the weight, measurements and any laser inscription against the paperwork.
- What is the return policy? Get it in writing, with enough time to have the stone checked by an independent appraiser.
The care chart
Hardness is a stone’s resistance to scratching on the Mohs scale. Dust is roughly 7, which is why softer stones lose their polish over time.
| Stone | Hardness | Everyday ring? | Cleaning |
|---|---|---|---|
| Diamond | 10 | Yes | Warm soapy water, soft brush |
| Ruby & sapphire | 9 | Yes | Warm soapy water; avoid ultrasonic if filled |
| Alexandrite | 8.5 | Yes | Warm soapy water |
| Spinel | 8 | Yes | Warm soapy water |
| Emerald | 7.5–8 | With care | Lukewarm water only; no ultrasonic or steam |
| Paraíba tourmaline | 7–7.5 | With care | Warm soapy water; no steam |
| Tanzanite | 6–7 | Occasional | Warm soapy water; no ultrasonic or steam |
| Pearl | 2.5–4.5 | Occasional | Wipe with a soft damp cloth after wearing |
Store pieces separately so harder stones don’t scratch softer ones, and have prongs checked by a jeweler once a year.
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This guide is for general education and is not an appraisal or financial advice. Treatments, origins and naming conventions vary by laboratory; always rely on the report that accompanies a specific stone.
