Home » Gemstones: A Comprehensive Guide » Identifying a Precious Stone or Gemstone: A Comprehensive Guide

Knowing how to identify a precious stone or gemstone is becoming increasingly important, as imitations and lab-grown gemstones are getting better and better. In this guide, you’ll learn the difference between natural, synthetic, artificial and imitation gemstones, discover the main Brazilian gemstones (beryls and tourmalines) and learn how to use three simple tools: a 10x magnifying glass, a Chelsea filter and a torch. At the end, there’s an FAQ section covering the most common questions.

The main challenge today is knowing how to distinguish a natural gemstone from a synthetic one or an imitation.

Gemstones have captivated people with their colour and crystalline structure for thousands of years. The first stones were probably found in riverbeds, and as civilisations became increasingly interested in them, specific techniques for extracting them were developed.

Over time, the term ‘precious stone’ came to refer to a natural mineral prized for its beauty, valued for its rarity and resistant to wear and tear. The rarer the gemstone, the more expensive it tends to be, and it was this rise in value that spurred the production of imitations and synthetic gemstones.

Gemmology, a science linked to mineralogy and crystallography, was developed to evaluate and classify gemstones using objective criteria. Gemmologists use standardised equipment and procedures; therefore, when purchasing a high-value gemstone, the best course of action is to consult a gemmologist or request a report from a recognised laboratory.

Precious and semi-precious stones: what has changed

For a long time, only four stones were referred to as ‘precious’: the diamond, the emerald, the ruby and the sapphire. The rest were called ‘semi-precious’.

Today, gemmology prefers the term gem for any mineral (or rock, or organic material such as pearls and amber) prized for its beauty, durability and rarity. The distinction between ‘precious’ and ‘semi-precious’ has no scientific basis, is confusing and ultimately undervalues Brazilian gemstones of great beauty, such as aquamarine, opal, amethyst, chrysoberyl and alexandrite. In the trade, however, these terms are still used, which is why they remain in the text.

For a gemstone to have gemmological value, it generally needs to combine beauty, rarity, tradition, fashion and durability. Therefore, the idea that ‘only the four classic gemstones are worth a lot’ is incorrect: the price depends on colour, quality, size and demand, and a gemstone of a different variety, of excellent quality, can be worth more than a classic gemstone of inferior quality.

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Como identificar um diamante

Brazil’s main gemstones

Among the many gemstones found in Brazil, beryls and tourmalines stand out in particular. Here’s a look at each group.

1. Beryls

Beryl is a beryllium-aluminium silicate (Be₃Al₂Si₆O₁₈). It forms prismatic crystals with a hexagonal base, has a hardness of 7.5 to 8, a specific gravity of around 2.6 to 2.8, a vitreous lustre and weak cleavage, and may be transparent or translucent.

Pure beryl is colourless, and impurities give each variety its colour:

• Emerald: a green beryl, coloured mainly by chromium and, in many stones, by vanadium;
• Aquamarine: a blue or blue-green beryl, whose colour is due to iron;
• Morganite: a pink beryl, associated with manganese;
• Heliodor (or golden beryl): a bright, clear yellow or greenish-yellow beryl, containing iron;
• Goshenite: colourless beryl;
• Bixbita: the rare red beryl. Some sellers use the name ‘red emerald’, which many experts avoid as it can be confused with the green emerald.

Find out more about emeralds and aquamarines.

2. Tourmaline

Minerals in the tourmaline group are boron and aluminium silicates, with a variable composition due to substitutions by elements such as iron, magnesium, sodium, calcium and lithium. They form prismatic crystals, ranging from long and slender to thick, with a rounded triangular cross-section, a hardness of 7 to 7.5 and a specific gravity of between 2.9 and 3.2. They range from transparent to opaque, have a vitreous lustre and display vertical striations that are clearly defined, which help to identify them.

Tourmalines are named after their colour:

• Dravita: brown;
• Schorlite: black;
• Elbaíta: green;
• Rubellite: pink to red;
• Indicolite: dark blue;
• Acroite: colourless.

There are also bicoloured varieties, such as watermelon tourmaline (pink on the inside and green on the outside), and the valuable Paraíba tourmaline, which is a deep blue – known commercially as ‘neon blue’ or ‘electric blue’ – a colour associated with the presence of copper. See also tourmaline and Paraíba tourmaline.

Natural, synthetic, artificial or imitation gemstone?

To identify a gem, you need to know which category it belongs to:

• Natural gemstone: formed by nature. It may be treated (for example, by heating or with oil) and still be considered natural, but the buyer must be informed of any treatment;
• Synthetic (or laboratory-grown) gemstone: produced in a laboratory, with the same chemical composition, crystalline structure and physical and optical properties as the natural gemstone. In some cases, such as with synthetic turquoise and opal, additional compounds may be present;
• Artificial gemstone: man-made, but with no natural counterpart, such as cubic zirconia;
• Imitation: a material with a similar colour and appearance but a different composition and properties, such as glass, plastic and ceramics.

Synthetic gemstones

A synthetic gemstone is ‘real’ in the chemical sense: it is the same substance as a natural one, only produced in a laboratory. This is why it is the most difficult to identify, and often only a laboratory can distinguish it from a natural one. For some materials, such as diamonds, the market and laboratories now use the term ‘lab-grown’ instead of ‘synthetic’. If you’d like to find out more, read the post on the 4 diamond classifications, which also explains lab-grown diamonds.

Artificial gemstones

Synthetic gemstones are man-made and have no natural mineral counterpart. Among the best known, many imitate the diamond: cubic zirconia (zirconium oxide), yttrium-aluminium garnet (YAG), gallium-gadolinium garnet (GGG), fabulite (strontium titanate) and synthetic moissanite (silicon carbide).

Imitation gemstones

Imitations are replicas that mimic the colour and appearance of natural gemstones, but have very different physical properties and chemical composition. They are mainly made of glass, paste or rhinestones, earthenware, porcelain and plastic, and many are moulded rather than cut, which gives away their true nature to anyone examining them with a magnifying glass.

And what if there’s no gemmologist to hand?

Where to buy gemstones

Brazil has centres for the trade and cutting of gemstones, such as Teófilo Otoni, in the Mucuri Valley (MG), known as the ‘world capital of precious stones’, and Governador Valadares, also in Minas Gerais. According to a report in *Estado de Minas*, Teófilo Otoni is home to around 250 small gem-cutting businesses and receives rough stones from mining sites in eastern Minas Gerais, western Espírito Santo and southern Bahia. There are also trade fairs for the sector (the Teófilo Otoni International Gemstone Fair has been running since 1989), specialist shops and tours of mining areas.

When you are familiar with a supplier’s integrity, it becomes easier to buy certified gemstones. In other situations, you need at least a basic understanding to ensure you do not end up buying glass instead of a gemstone. Jewellers, jewellery designers and goldsmiths need to understand the principles of gemmology in order to present reliable work to their customers. An error in identification – such as setting glass sold as onyx – can result in financial loss and undermine the professional’s credibility.

Even for experienced professionals, today’s counterfeits have reached worrying levels of quality. Consequently, any gemmological analysis that is not thorough is prone to error, and the simple tests below serve as a screening tool rather than definitive proof.

Three simple and reasonably priced tools are a great help: a 10x magnifying glass, a Chelsea filter and a torch.

How to examine a precious stone or gemstone

1. Price

One of the first things to consider is the price. Prices vary depending on the seller and the region, and the market itself sets the price levels. Do plenty of research before buying. If the price is well below market value, be wary, because nobody can work miracles. The price does not prove authenticity, but it is the first warning sign.

2. Using a magnifying glass

A jeweller’s loupe (10x or higher) reveals what lies inside the gemstone. Hold the loupe close to your eye and move the gemstone away until you can focus, preferably with the stone well lit. What to look for:

• Inclusions: natural gemstones usually have small internal flaws, growth marks and irregular patterns. Emeralds, for example, almost always have inclusions (the ‘garden’, as it is known) and fractures. But bear in mind: not every natural gemstone has flaws, and synthetic ones also have their own characteristics. The absence of inclusions is a reason to investigate further, but it is not a deciding factor on its own;
• Bubbles: Perfectly round bubbles inside the stone suggest it is glass. However, natural gemstones may also contain gas bubbles, which are usually irregular in shape or found within cavities containing liquid (known as three-phase inclusions). You should therefore assess the shape and context of the bubble;
• Treatments: Oil in fractures appears as a swirling glow, and dye may accumulate in the fractures.

3. Using the Chelsea filter

The Chelsea filter was developed in 1934 by Anderson and Payne at the London Chamber of Commerce’s gemmological testing laboratory, with the help of students from Chelsea College of Science and Technology. It was originally used to distinguish emeralds from imitations, such as green glass, green tourmaline and peridot.

In the early days, only chromium-coloured beryls were considered to be emeralds, whilst those coloured by vanadium were treated as a different type of beryl. Today, the definition is a matter of debate, and many laboratories also accept stones containing vanadium. Over time, this criterion has also come to be used to distinguish other natural gemstones from imitations.

How the Chelsea filter works

The Chelsea filter allows only two bands of light to pass through: deep red (around 690 nm) and greenish-yellow (around 570 nm). Light that is not transmitted appears dark. To use it, shine a strong incandescent lamp or torch on the stone and look through the filter, holding it a few centimetres from your eye. LED light is not suitable for this test.

A genuine emerald, coloured by chromium, usually appears pinkish or reddish, whilst imitations such as green glass do not usually exhibit this red reaction. However, there are important limitations:

• The synthetic emerald also appears reddish, because it too contains chromium. The filter does not distinguish between natural and synthetic emeralds. This requires a gemmologist and, often, a laboratory;
• Emeralds coloured mainly by vanadium may show little or no reaction;
• The filter is a screening tool, not a diagnostic tool. If in doubt, further gemmological tests are required.

Synthetic blue spinel and other gemstones

In some cases, the filter is very helpful: synthetic blue spinel, coloured with cobalt, exhibits a reddish reaction, which distinguishes it from similar blue gemstones, such as aquamarine and blue topaz, which absorb red light and do not exhibit the same reaction.

In gemstones such as tanzanite and alexandrite, imitations and synthetics may also react differently from natural stones, displaying a brighter red or, conversely, showing no reaction at all. The photos below are for illustrative purposes only, and the shades seen through the filter vary considerably. The important thing is to understand the principle behind the analysis.

4. Using the torch

A torch helps you to examine the brilliance, transparency and inclusions, and is essential when using a loupe and a Chelsea filter. Rotate the stone in the light and observe how it reflects it: a well-cut gemstone usually reflects light more vividly. Dark spots, cracks and dull areas may indicate fractures, poor-quality material or poor cutting, and do not necessarily indicate a fake.

Treatments and reports

Many gemstones undergo treatments to improve their colour or clarity, such as heating, filling cracks with oil or resin, and irradiation. This is common practice and accepted provided the buyer is informed, but it does affect the value of the item and how it should be cared for.

When making high-value purchases, always ask for a gemmological laboratory report and check that it specifies the type of gemstone, whether it is natural or lab-grown, and whether it has been treated. Laboratories use chemical analyses and advanced instruments – which are not available at home – to distinguish natural gemstones from synthetic ones.

Gemmology: an almost essential subject

Those who work with jewellery (designers, gemstone dealers, goldsmiths and jewellers) benefit greatly from an understanding of gemmology. Gemmology studies the physical and chemical properties of materials of gemological value, whether organic or inorganic, used as personal adornment or decoration. It also studies the origin, composition and optical properties of gemstones, the appropriate cut for each gem, and is concerned with distinguishing between natural, synthetic and imitation gemstones, as well as the treatments used to enhance their appearance and value.

Gemstones are substances of aesthetic value, such as colour, shape (cut) and durability, taking into account their physical and chemical characteristics and properties.

The specialist is the gemmologist. In Brazil, the Federal University of Espírito Santo (UFES) offers an undergraduate degree in Gemmology (a four-year bachelor’s degree, affiliated with the Centre for Legal and Economic Sciences in Vitória): gemologia.ufes.br. There are also short courses in gemmology, and it is worth researching the institution’s reputation thoroughly before enrolling.

Even with a good knowledge of the subject, an accurate gemmological analysis requires suitable equipment and instruments. A knowledge of gemmology helps with the initial screening, but it is no substitute for a gemmologist.

See also

• The 4 basic diamond grades
• Emerald: characteristics, varieties and quality
• Ruby: characteristics, colours and cutting
• Sapphire: characteristics and properties
• Aquamarine stone
• Amethyst: characteristics, trade and valuation
• Topaz: history and characteristics
• Tourmaline and Paraíba tourmaline
• Opala
• Malachite

Sources consulted

• Wikipedia: Chelsea filter
• Emeralds.com: basic ways to identify emeralds
• GIA: frequently asked questions about treated and lab-grown gemstones
• Estado de Minas: Teófilo Otoni, the gemstone capital of the world
• UFES: Gemmology course

Written by Willy Sievert

Editor of Bahia.ws since its founding in 2009. Based in Salvador, he travels frequently across the Northeast and all of Brazil to keep the guides up to date. Learn more about Willy Sievert →

Published on 29/01/2023 · Updated on 09/10/2026

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