What is a Gemstone? Definition, Colour and Durability

A gemstone is a natural material that has been cut and polished for use in jewellery or decorative objects. This article explains what qualifies as a gemstone, what makes one more valuable than another, where colour comes from and why hardness and toughness are not the same thing.

Definition of a Gemstone

A gemstone is a natural crystal, rock, mineral or organic material that has been cut and polished for use in jewellery or decorative objects.

To be classified as a gemstone, the material must exhibit beauty, durability and rarity, with colour often being the most important visual characteristic.

Transparent to translucent gemstones such as ruby, topaz, peridot and aquamarine are typically cut from a mineral's crystals. Opaque gemstones usually come from rocks or minerals. Examples include lapis lazuli, labradorite, bloodstone and jasper.

Organic gemstones come from living things rather than geological processes. Pearl, amber, coral and jet are the best known.

Evidence of our fascination with gemstones can be found throughout history, and today, our bond with these natural curiosities is stronger than ever.

Regardless of age or culture, we're captivated by their colours, how they interact with light, and the transformation that occurs once a gemstone has been cut and polished.

Gemstones are mentioned in texts of many religions and have long been used in rituals and ceremonies. In ancient Egypt, they were used as part of the complex burial rituals of the pharaohs because they were believed to help ease the soul into the afterlife.

In many ancient cultures, people were often buried with their most prized possessions, which included gemstones. This ritual has been invaluable to historians and archaeologists.

For thousands of years, the finest gemstones have been sought after by those for whom money is no object. Jewellery and accessories featuring diamonds, emeralds, rubies and sapphires can be found in collections around the world. Many feature gemstones considered priceless because of their size, rarity or beauty.

The Green Vault in Dresden, the Smithsonian in Washington D.C, the Armoury Palace in Moscow and the Tower of London, home to the British Crown Jewels, hold some of the most beautiful gemstones the world has ever seen.

What Makes a Gemstone Valuable?

Several factors are taken into account when classifying a gemstone. Although beauty, durability and rarity are the primary characteristics, without beauty, the other two mean very little.

What causes the value of one gemstone to increase can have the opposite effect on another. Rubies, sapphires and emeralds are prized for their colour, yet most diamonds are colourless.

The finest aquamarine can command an exceptional price if inclusion-free, while some varieties of quartz increase in value because of inclusions.

Clarity is taken into consideration when determining the value of certain gemstones, but it's not always relevant and depends on the type of stone. A fine aquamarine can be almost transparent and void of inclusions, yet it's incredibly rare to find a flawless emerald.

Flawless describes a gemstone that's completely transparent and free of inclusions, cracks and other anomalies.

The Terms Precious and Semi-Precious

Gemstones were once described as precious or semi-precious. Diamond, ruby, sapphire and emerald were the only precious gemstones. Everything else was semi-precious.

Gemmologists no longer use these terms because they create a misleading distinction between gemstones. 'Precious' suggests that one stone is inherently more valuable than another.

A gemstone’s value depends on its grade, rarity and market value. A high-grade red beryl or tsavorite garnet can be worth far more than a sapphire of a similar grade. Some stones once described as 'semi-precious' are far more scarce than diamond, and can be considerably more expensive, depending on the grade.

Although these terms are still sometimes used, mostly in jewellery shops and online, they are discouraged by the International Coloured Gemstone Association and the World Jewellery Confederation.

A Gemstone's Colour

The colour of a gemstone will often be its most important feature. It can also be the least reliable for accurate identification. Identifying a gemstone based on colour alone has led to countless mistakes being made.

Colour is determined by three main factors: the mineral's chemical composition, crystal structure, and natural defects and inclusions. The slightest change in any of them can bring about a change in colour, which is why some gemstones of the same mineral variety can be found in a wide range of colours.

In some minerals, colour comes from an element that's an essential part of the chemical composition, so the stone is always the same colour. Peridot is always green because of the iron in its structure. In others, the pure mineral is colourless and its colour comes from impurities.

Quartz is colourless when pure, yet trace amounts of iron plus exposure to radiation give amethyst its colour.

The largest group of gemstones are those which are mostly colourless in their purest form. These stones tend to be translucent or transparent.

A fine-grade gemstone must exhibit good depth of colour without being too pale or too dark. Colour should be uniform throughout, which can be an issue in some gemstones.

In amethyst, the shade of colour often varies within the same crystal. This characteristic, known as colour zoning, can be subtle or prominent. Banded amethyst (also known as chevron amethyst) is the most extreme example.

Hardness, Toughness, Stability

Durability is another factor that determines the suitability of a crystal, rock or mineral to be classified as a gemstone. This characteristic is comprised of three parts: hardness, toughness and stability.

Hardness and toughness are often looked upon as being the same, but they're different.

Hardness relates to scratch resistance, meaning how easily one gemstone can be scratched by another. The Mohs scale of mineral hardness is widely used for this purpose.

Toughness relates to a gemstone's resistance to cracking, chipping, pressure or breaking. It can also affect how easy it is to facet.

Although diamond may be the hardest mineral, it's not the toughest. Diamond grades 10 on the Mohs scale, whilst nephrite jade grades 6 to 6.5. However, nephrite is tougher than diamond because of its strong interlocking network of fibrous crystals.

Pearl grades 3, yet is very tough. If dropped from height onto a hard surface, there's a good chance it won't break. Diamond is extremely hard but not particularly tough. It's difficult to scratch but can fracture or cleave if struck along a cleavage plane, whereas nephrite and pearl can absorb impact more effectively.

Topaz is another hard mineral that grades 8 on the Mohs scale. Despite its hardness, topaz is fragile and can chip easily. It's also sensitive to heat and pressure, both of which can cause it to crack.

When searching for information online regarding toughness in relation to rocks, minerals or gemstones, most search results reference the Mohs scale of hardness. However, the Mohs scale is only used to determine hardness or scratch resistance, not toughness.

It can be quite difficult to find information on toughness without using the correct geological term, which is 'tenacity'.

Stability refers to a gemstone's resistance to chemicals, heat, light and environmental exposure that can cause fading, alteration or structural damage.

Using turquoise as an example, this natural mineral is delicate and porous, so can be easily damaged by pollutants in the atmosphere. It can also be damaged by moisture. Another example is amethyst, which will fade with exposure to sunlight.

Faceted Gemstones and Cabochons

A gemstone can be polished as a cabochon or have facets cut into it. Both aim to maximise beauty, which increases value.

A cabochon is a gemstone polished to produce a smooth, rounded upper surface and a flat base. The technique is used mainly on opaque stones, or those that exhibit minimal translucence.

The domed shape enhances surface colour and markings as well as optical phenomena such as chatoyance, schiller, asterism and play of colour.

Highly translucent or transparent gemstones tend to be faceted. This involves cutting a series of flat reflective faces called facets into the surface. Facets enhance a gemstone's ability to reflect light, which improves colour, sparkle and brilliance.

Article Pictures

The second photo in our article is black rutile inclusions in quartz.

The banded amethyst stones are from our collection.

The topaz crystal is courtesy of Stan Celestian.

Pop-up photos

Ruby and Peridot: Courtesy of Stan Celestian. Jasper: Courtesy of James St. John. These photos link to the original images on the photographic website Flickr. You may only be able to view them if you join. Joining Flickr is free, with no additional charges.

The remaining pop-up photos feature pieces from our own collection.