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How to Identify a Precious Stone
Bonnot Paris
April 29, 2026
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5 min read
Bonnot Paris · Guide to Precious Stones
How to identify a genuine precious stone?
Sapphire, ruby, emerald or diamond: the gemologists' secrets for authenticating and assessing the quality of a gemstone — by Maison Bonnot Paris.
A deep sapphire, a vivid ruby, a captivating emerald or a sparkling diamond: the world of gemstones is the stuff of dreams. But how does one tell a genuine precious stone from an imitation or a synthetic stone?
Faced with an abundance of offerings and sometimes complex artificial treatments, a legitimate question arises. As a creator of bespoke jewellery, Maison Bonnot Paris reveals the secrets gemologists use to identify the authenticity and quality of a gemstone.
01
Vocabulary
Understanding the term "Precious Stone"
First and foremost, it is important to clarify a point of terminology. Historically, only four gemstones hold the designation "precious stone" — a status recognised by international gemology for centuries.
All other stones (aquamarine, tourmaline, garnet, etc.) were known as semi-precious or fine stones. Although French legislation now groups all these marvels under the single term "gemstones", the quality criteria and evaluation methods remain specific to each stone.
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Diamond
Mohs 10 · The hardest
The absolute benchmark in fine jewellery. Its incomparable hardness and unique fire make it the quintessential traditional engagement stone.
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Sapphire
Mohs 9 · Highly durable
A symbol of fidelity and wisdom. Available in blue, pink, yellow or padparadscha — an infinite palette for singular creations.
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Ruby
Mohs 9 · Stone of desire
The ultimate expression of passionate red. Fine untreated rubies frequently surpass diamonds of equivalent carat weight in value.
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Emerald
Mohs 7.5–8 · Singular green
Its velvety green hue and natural "garden" make it a stone of unparalleled character, the favourite of Cleopatra and the great jewellers.
02
Visual identification
Observation: the signs that never lie
Without laboratory equipment, absolute certainty is impossible. Yet careful observation can reveal telling indicators. Here are three tests accessible to all for assessing a gemstone with the naked eye.
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Temperature
Immediate test
A genuine precious stone feels cool to the touch, even after prolonged contact with the skin. Glass or plastic quickly takes on the ambient temperature. A stone that remains cold is a first positive signal.
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Inclusions
Under ×10 loupe
Nature is not perfect. Almost all natural stones contain inclusions. Under a jeweller's loupe (×10), a stone that appears "too perfect" is often a sign of synthesis or glass. The emerald makes its inclusions part of its very identity.
03
Evaluation criteria
The golden rule of the 4Cs
To assess the value and quality of a precious stone, experts rely on the international rule of the 4Cs. These four criteria apply to all gemstones — from the diamond to the emerald.
C1
Couleur · Color
The number one criterion for coloured stones. Hue, tone (light to dark) and saturation (intensity) are all assessed. It accounts for approximately 70% of the value of a ruby, a sapphire or an emerald.
C2
Clarté · Clarity
The absence or presence of inclusions visible to the naked eye or under a loupe. For the diamond, absolute purity is ideal. For the emerald, inclusions are part of its identity.
C3
Taille · Cut
The way in which the stone has been faceted to reveal its maximum brilliance. A poor cut will render a beautiful stone dull. An expert jeweller will be able to identify a quality cut at a glance.
C4
Poids · Carat
The universal unit of measurement: 1 carat = 0.20 grams. The heavier the stone, the rarer it is — and its value increases exponentially, not linearly, beyond certain thresholds.
04
The ultimate guarantee
The gemmological certificate: the only certainty
Let us be clear: even the most trained eye can today be deceived by certain highly sophisticated synthetic stones. The only way to formally identify a precious stone is the gemmological certificate.
Issued by an independent, internationally recognised laboratory, this document is the identity card of your gemstone. It attests that it is natural and specifies whether it has undergone any treatments — such as the classic heat treatment for a sapphire or oiling for an emerald.
The reference laboratories
The three most recognised laboratories in the world are the GIA (Gemological Institute of America), the GRS (GemResearch Swisslab) and the ALGT (Antwerp Laboratory for Gemstone Testing). At Bonnot Paris, all our precious stones are accompanied by a certificate from one of these independent bodies.
05
Our promise
The Bonnot Paris commitment: transparency with no middlemen
At Bonnot Paris, we have made a commitment to absolute transparency. Because a bespoke piece of jewellery deserves an exceptional stone, we leave nothing to chance — from the mine to your finger.
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Direct sourcing
At the source · Sri Lanka · India · Thailand
We are the only fine jewellery house to maintain our own buying offices directly in producing countries, with no intermediaries whatsoever.
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Zero middlemen
Full traceability · Fair pricing
By purchasing our gemstones directly at the source, we guarantee the traceability of your precious stone and the fairest price on the market.
06
Frequently asked questions
Everything you have ever wondered about precious stones
How can you tell a real precious stone without any equipment?
Several indicators can guide you: the temperature to the touch (a genuine stone feels cold), the presence of natural inclusions visible under a loupe, and the way it interacts with light. However, these observations are no substitute for a gemmological certificate issued by an independent laboratory such as the GIA or GRS.
What are the 4 precious stones recognised by gemmology?
The four historically recognised precious stones are the diamond, the sapphire, the ruby and the emerald. All other gems are grouped under the term « pierres fines » in France, even though their beauty and value can be equally remarkable.
What is the 4C rule for a precious stone?
The 4Cs are the four international criteria for evaluating a gemstone: Colour (hue, tone, saturation), Clarity (presence of inclusions), Cut (quality of the faceting) and Carat (weight, 1 carat = 0.20 grams). For coloured stones such as the sapphire or the ruby, colour accounts for approximately 70% of the value.
Why choose Bonnot Paris for purchasing a precious stone?
Bonnot Paris is one of the only fine jewellery houses to maintain its own buying offices directly at the source, in Sri Lanka, India and Thailand. This intermediary-free sourcing guarantees stone traceability and competitive pricing. All our stones are accompanied by an independent laboratory certificate. Book an appointment at our atelier to discover our selection.
Ready to choose your precious stone?
Stop choosing your gemstone in the dark. Book an appointment with Maison Bonnot Paris to discover our certified collection and create the jewel of your dreams together.
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