The Roho gem guide

A gemstone is easy to love and harder to judge. This guide explains what we look at before we buy a stone, so you can judge one yourself, ask the right questions and know exactly what you are paying for.

How to judge a gemstone

For coloured stones, colour comes first, then clarity, then cut. Always look in daylight or neutral light: warm indoor light flatters some stones and dulls others.

Sapphire

Sapphire is corundum, 9 on the Mohs hardness scale and the September birthstone. Look for even colour that is neither so dark it turns inky nor so light it looks washed out. Cornflower blue is lighter and brighter, royal blue deeper; neither is better, it is a matter of taste. A fine sapphire is clean to the eye face-up, though fine natural silk is normal. A good cut returns light across the whole face; a pale "window" in the centre, where you can see straight through, means the stone is cut too shallow. Pink, yellow, orange and purple sapphires are judged the same way.

Ruby

Ruby is red corundum, also Mohs 9, and the July birthstone. The most sought-after colour is a pure to slightly purplish red that stays bright in different light. Rubies are rarely flawless, so expect small inclusions; what matters is that they don't spoil the colour or the brightness. Be careful with very cheap, very clean-looking rubies: many are glass-filled (see below).

Emerald

Emerald is green beryl, 7.5 to 8 on the Mohs scale and the May birthstone. Nearly every emerald has natural inclusions, called the jardin (garden), so judge colour first: a lively, saturated green matters more than perfect clarity. The emerald cut, with its cut corners, helps protect this more brittle stone. Avoid hard knocks, heat and ultrasonic cleaners.

Pearl

Pearls are judged on lustre first: a sharp, bright reflection on the surface. Then surface (fewer spots and bumps), shape (round is rarest, but drops and baroque shapes have their own charm), size and colour. Almost all pearls sold today, Tahitian pearls included, are cultured: grown by a living oyster around a nucleus placed by a pearl farmer. Genuine means a real pearl grown by an oyster, not an imitation made of glass or plastic. Pearl is the June birthstone.

Treatments, explained

Many gemstones are treated to improve their colour or clarity. Treatment is not a problem in itself; not being told about it is. Treatment affects price, durability and care, so every Roho listing states it.

• Heat: the most common treatment for sapphire and ruby. Heating improves colour and clarity, is permanent and needs no special care. Heated stones cost less than comparable unheated ones.
• Oil: the traditional treatment for emerald. Colourless oil fills tiny surface-reaching fissures so they show less. "Minor oil" means a small amount. Oil can slowly dry out over many years and can be removed by ultrasonic or steam cleaning, so clean emeralds gently.
• Glass filling: lead glass is used to fill fractures in low-grade ruby and sapphire. It makes the stone look much cleaner, but the glass is soft: it can be damaged by acids, household chemicals, a jeweller's torch or ultrasonic cleaning. Glass-filled stones should cost far less and must always be labelled. We label ours clearly.
• Untreated (unheated, no oil): the stone has only been cut and polished. Untreated stones are rarer and cost more, so ideally the claim is confirmed by an independent laboratory.

Lab reports

An independent gemmological laboratory report confirms what a stone is, whether it is natural or lab-grown, and whether there are indications of treatment. Some reports also give an opinion on geographic origin. The laboratories you will see on our listings:

• GIA (Gemological Institute of America): widely recognised for diamonds and coloured stones.
• AGL (American Gemological Laboratories, New York): known for coloured stones, treatment and origin.
• GFCO (Switzerland): coloured stone and diamond reports.
• AIGS (Asian Institute of Gemological Sciences, Bangkok): coloured stones and pearls.
• IGI (International Gemological Institute): widely used for diamonds, including lab-grown diamonds.

A report describes a stone; it does not grade its beauty or predict its value, so colour in daylight still matters most. "No lab report" on a listing means we have identified and checked the stone in-house and no independent report is included. We can arrange one on request and tell you the cost and timing first. For any stone sold as unheated or untreated, if an independent lab finds a treatment we did not disclose, you get a full refund.

Natural or lab-grown?

A natural gemstone formed in the earth. A lab-grown stone has the same chemistry and physical properties but is made in a laboratory, in weeks rather than millions of years. Both are real gemstones; they are simply different products at very different prices, and you should always be told which one you are buying. Our coloured gemstones are natural and earth-mined. Lab-grown diamonds, such as CVD diamonds, are always labelled lab-grown on our site.

Caring for silver and gold

• 925 sterling silver is 92.5% silver, alloyed for strength. Our silver pieces are nickel-free. Plating (yellow gold, rose gold or rhodium) wears with friction, so put jewellery on after lotion and perfume, take it off before swimming or showering, wipe it with a soft cloth and store it in its pouch.
• 18K gold is 75% gold and does not tarnish. Clean it with warm water, mild soap and a soft brush.
• Whatever the metal, keep emeralds, pearls and glass-filled stones away from ultrasonic and steam cleaners.

Birthstone chart

Month	Birthstone
January	Garnet
February	Amethyst
March	Aquamarine
April	Diamond
May	Emerald
June	Pearl
July	Ruby
August	Peridot
September	Sapphire
October	Opal
November	Topaz
December	Tanzanite

For more stones by month and zodiac sign, see our full birthstone chart.

A note on value

We sell gemstones to wear and enjoy, not as investments, and we make no claims about future value. Buy a stone because you love its colour.

Questions about a stone, a treatment or a report? Contact us; we usually reply within a few hours.