Durability, Stability, & Cleaning of Gemstones

Durability, Stability, & Cleaning of Gemstones

Jewellery is one of our most intimate and cherished accessories, and as such we should take all steps to look after it. As well as following the advice of general wear, certain jewellery will benefit from additional knowledge and care. Gem and mineral hardness is measured on the Mohs scale. The numbers are based on the relative ease or difficulty with which one mineral can be scratched by another. But the Mohs scale is deceptive. The steps between the minerals are not evenly spaced. For example, diamond is only one number away, but it’s many times harder than gems in the corundum family.

The advice for the following materials are listed alphabetically:

  • Amber
  • Amethyst
  • Aquamarine
  • Chrysoberyl and Alexandrite
  • Diamond
  • Emerald
  • Garnet
  • Jade
  • Opal
  • Pearls
  • Peridot
  • Ruby
  • Sapphire
  • Spinel
  • Tanzanite
  • Topaz
  • Tourmaline
  • Turquoise
  • Zircon

The following advice contains details about both natural and treated gemstones. We at Sayer & Sons will never knowingly sell any gemstone which has been treated in such a way that prevents it from being worn and enjoyed in the same way that untreated gemstones should be worn. We maintain the standards outlined by CIBJO for the proper disclosure on precious stones and metals.

Amber
Amber is soft compared to gem minerals and many other organics. It falls between 2 and 2.5 on the Mohs scale. Amber also has poor toughness, so care must be taken when wearing amber not to scratch and damage it.

Stability
Amber burns at relatively low temperatures. With prolonged exposure to light it might darken, and it can be damaged by chemicals.

Cleaning
The only safe way to clean amber is with warm, soapy water. Be sure to use a mild soap.

Treatment and durability considerations
The dyes used to colour amber are not stable and can change or fade under certain circumstances.

Amethyst
Common substances like sand and dust are composed mostly of quartz, the mineral species that includes the purple variety amethyst. So, while amethyst is a great gemstone for use in jewellery, it should be treated with the proper care to prevent scratching and other damage. Amethyst rates a 7 on the Mohs scale and has good toughness, so it is suitable for all jewellery types. This includes rings as long as the wearer understands the limits of its hardness. A ranking of 7 on the Mohs scale means amethyst is a durable jewellery gem as long as proper precautions are followed to prevent scratching.

Stability
Abrupt temperature changes can cause amethyst to fracture. Some amethyst colour can fade with prolonged exposure to intense light. Amethyst can also be damaged by hydrofluoric acid, ammonium fluoride, and alkaline solutions.

Cleaning
Amethyst can be safely cleaned with warm soapy water. Ultrasonic cleaners are usually safe except in the rare instances where a stone is dyed or treated by fracture filling. Steam cleaning is not recommended, and amethyst should not be subjected to heat.

Aquamarine
Aquamarine is 7.5 to 8 on the Mohs scale, so it is a durable gemstone for jewellery as long as it is treated with care to protect it against scratching and hard knocks.

Stability
Heat exposure is not recommended for aquamarine, but the colour is stable against light exposure. Aquamarine can be attacked by hydrofluoric acid.

Cleaning
Warm soapy water is always a safe cleaning method for aquamarine. Cleaning by ultrasonic and steam cleaners is usually safe unless the stone has liquid inclusions or fractures.
Rarely, aquamarine might be fracture filled. These stones should only be cleaned with warm soapy water.

Chrysoberyl and Alexandrite
Alexandrite is relatively hard—8.5 on the Mohs scale. It has excellent toughness and no cleavage, which is a tendency to break when struck. This makes it a good choice for rings and other mountings subject to daily wear.

Stability
Alexandrite is stable under normal wearing conditions, which means it’s resistant to the effects of heat, light, and common chemicals.

Diamond
Something the Mohs scale does not show, but that’s equally important to the diamond industry, is that diamond can also scratch any of the precious metals used for settings. That means a diamond that’s loose in its setting can wear through a prong over time.

Toughness
Any stone, including a diamond, will fracture if it’s hit hard enough in the right place. Toughness is a measure of how well a gem can survive an impact and resist breaking, chipping, or cracking. Diamonds are tougher in the directions where the atoms are bonded tightly together, less tough where they’re not so tightly bonded. The weakest directions are the ones where the atoms are farthest apart. It’s easier to break a diamond in those directions, which are called cleavage directions. A cutter can split a rough diamond by hitting it sharply in the cleavage direction. Even after cutting, a hard blow can still cleave or fracture a diamond. This can happen during the setting process, when it’s being worn, or when it accidentally strikes another object.

Stability
Stability is a term that describes how well a diamond resists temperature changes and chemicals. Diamonds are very stable. They’re invulnerable to virtually all acids, for one thing. The cutting process generates a lot of heat, but diamonds usually endure intact. Situations that are more threatening to a diamond’s stability are those that involve sudden and extreme temperature changes. Those changes can cause thermal shock and create new fractures and cleavages or cause existing ones to spread. Diamonds will burn at about 1562°F (850°C). House fires and jewellers’ torches can reach that temperature. During jewellery repair, the bench jeweller can use certain methods to protect the diamond from the torch’s high-temperature flame.

Cleaning
Diamonds can be safely cleaned with lint-free cloths, commercial jewellery cleaning solutions, and household detergents. Harsher cleaning methods are not recommended for household use. These include powdered abrasive household cleansers, ultrasonic cleaners, and steam cleaners.

Emeralds
Emerald is 7.5 to 8 on the Mohs scale and has fair to good toughness, making it a stone that requires more care in wearing than ruby or sapphire. Even so, emeralds are beautiful stones for all types of jewellery and with proper care will last for generations.

Stability
Heat can damage emeralds, especially by extending existing fractures. Light and chemicals can cause the oils, resins, and polymers used to fill surface-reaching fractures to alter in appearance or deteriorate.

Cleaning
Some estimates state that 90 percent or more of emeralds are fracture-filled. Since the great majority of fashioned natural emeralds contain filled fractures, it’s risky to clean them ultrasonically or with steam. Ultrasonic vibrations can weaken already-fractured stones, and hot steam can cause oil or unhardened resin to sweat out of fractures. Using warm, soapy water coupled with gentle scrubbing is the safest way to clean emeralds.

Garnets
Depending on type, garnet hardness ranges from 6.5 to 7.5 on the Mohs scale. For example, almandine, pyrope, spessartine, and tsavorite are on the harder side while demantoid is a bit softer.
Garnets have fair to good toughness, making them durable enough for all jewellery styles as long as they are treated with the proper care. Garnets should not be subjected to any hard blows or rough wear.

Stability
Garnets are stable to light and chemicals. They can be attacked by hydrofluoric acid.

Cleaning
Warm soapy water is always safe for cleaning garnets. The ultrasonic cleaner is usually safe except for stones that have fractures. Steam cleaning is not recommended.

Treatment and durability considerations:
Garnets might rarely be treated by fracture filling. Only warm soapy water should be used to clean fracture-filled stones.

Jade
Jadeite is 6.5 to 7 and nephrite is 6 to 6.5 on the Mohs hardness scale, so some care should be taken to avoid scratches. However, both of these gemstones have exceptional toughness and are very resistant to breaking or chipping.

Stability
Heat from a jeweller’s torch can harm jade. Jade is stable to light, and it can be affected by warm acids.

Cleaning
Ultrasonic cleaners and steam cleaners are safe for untreated jade. But jade might be treated by dyeing, bleaching and impregnation, coatings, or heat treatment, so warm, soapy water is always a safe choice.

Moonstone
Moonstone falls between 6 and 6.5 on the Mohs scale. It also has poor toughness.

Moonstone has two cleavage directions, a trait it shares with the other members of the feldspar family of minerals. Cleavage is a direction of atomic weakness that makes a crystal more prone to breaking. Cleavage can affect durability, so manufacturers usually set moonstone into pendants, earrings, and pins rather than rings, which are more likely to receive knocks and bumps.

Stability
High heat or sudden temperature change can cause breaks in moonstone. It is stable to light but is harmed by exposure to hydrofluoric acid.

Cleaning
Warm soapy water is the only recommended substance for cleaning moonstones. Ultrasonic and steam cleaners are never recommended. 

Opal
Opal hardness is variable depending on its exact composition and formation conditions, and ranges from 5 to 6.5 on the Mohs scale. Its toughness is very poor to fair, making opal a gem that is suitable for jewellery but requires care when wearing so as to not scratch or break the stone.

Stability
Opal is generally stable, but heat from intense light can cause fracture lines called “crazing.” High heat or sudden temperature changes can also cause opal to fracture. Opal is attacked by hydrofluoric acid and caustic alkaline solutions.

Treatments
Opals can be treated by impregnation with oil, wax, or plastic, and by surface modifications called sugar treatment and smoke treatment.

Cleaning
The only safe way to clean opal is with warm, soapy water.

Pearls
Pearl is ranked 2.5 on the Mohs hardness scale, which means it’s very soft and easily scratched or abraded. But with reasonable care, pearl jewellery can be a lasting treasure. Pearl’s toughness is usually good, but aging, dehydration, and sometimes excessive bleaching during initial processing might make some pearls more fragile.

Stability
Pearls are a natural product of an organic process; as a consequence they require some special care. It is advised that you should avoid all exposure to water (do no shower or bath whilst wearing them) and chemicals (hairspray, perfume, and beauty products). Naturally occurring oils of the skin can affect the lustre and structure of the pearl, as such it is advised that they are not worn continuously, and to be put on after any such beautification products have had ample time to apply.

When strung as a necklace or bracelet the material on which they are strung is specially designed for strength and durability. Where the thread meets the clasp a special wound wire further strengthens the thread. Although strong this thread will elongate and loosen, and the metal wire will deteriorate and become brittle. It is advised that pearl necklaces should be restrung annually. This will ensure that thread doesn’t become a way for moisture to enter the interior of the pearl, and the metal wire is adequately firm. If the necklace consist of a double or triple row when one row requires restringing it is best practice to rethread all the rows. This will keep the lengths correct, meaning it appears normal, and also means it can remains at the workshop less frequently. When a necklace is freshly restrung it is normal (especially so for knotted pearls) that it appears wiggly. After wearing from a short duration it will naturally straighten – it does not require pulling, or being weighted – normal wear is the best solution for straightening freshly strung pearls.

When set as a ring it should be known that their structure and method of setting prevent them from being a ‘hard wearing’ gemstone. They can never be set as firmly as other materials and often rely on additional adhesives for their sturdiness. Along with general care one should take extra precautions to prevent exposure to water and chemicals (as for example when washing hands).

Cleaning
A soft svelte cloth is best used to clean any item of jewellery which contains pearls. Occasionally using a small amount of pure warm water is recommended.

Peridot
Peridot is 6.5 to 7 on the Mohs scale. It has fair to good toughness. Peridot is durable enough for jewellery wear but should be worn with some care so as not to scratch it or subject it to hard blows that can fracture it.

Stability
Rapid or uneven heat can cause peridot to fracture. Peridot is stable to light but is easily attacked by sulphuric acid, and less easily by hydrochloric acid. It can also be attacked over a long period of time by acid perspiration.

Cleaning
Warm, soapy water is the best method for cleaning peridot. Ultrasonic and steam cleaners are not recommended.

Ruby
Corundum (ruby and sapphire) is relatively hard—9 on the Mohs scale. It has excellent toughness and no cleavage, which is a tendency to break when struck. This makes it a great choice for rings and other mountings subject to daily wear.

Stability
Corundum is stable under normal wearing conditions, which means it’s resistant to the effects of heat, light, and common chemicals. Boric acid powder will etch the surface of even untreated stones. Fracture-filled, cavity-filled, and dyed stones can be damaged by even mild acids like lemon juice.

Cleaning
Warm soapy water is always safe. Ultrasonic and steam cleaners are usually safe for untreated, heat-treated, and lattice diffusion treated stones. Fracture-filled, cavity-filled, or dyed material should only be cleaned with a damp cloth.

Treatment and durability considerations
Untreated ruby and even heat-treated ruby are very durable. Stones that have undergone lattice diffusion treatment have varying degrees of treated-colour penetration. In some stones, the treated colour penetrates the entire stone, while others have very shallow treated-colour penetration. For stones with shallow colour penetration, surface damage or re-cutting can remove colour.

Today’s fracture-filled stones have surface-reaching fractures filled primarily with a high-lead content glass. There are large numbers of these treated rubies in the market and they require greater care than untreated, heat-treated, or lattice diffusion treated ruby. The glass can be damaged through contact with a variety of chemicals. Even relatively mild substances like lemon juice can cause changes in the high-lead content glass.

Sapphire
Corundum (ruby and sapphire) is relatively hard—9 on the Mohs scale. It has excellent toughness and no cleavage, which is a tendency to break when struck. This makes it a great choice for rings and other mountings subject to daily wear.

Stability
Corundum is stable under normal wearing conditions, which means it’s resistant to the effects of heat, light, and common chemicals. Boric acid powder will etch the surface of even untreated stones. Fracture-filled, cavity-filled, and dyed stones can be damaged by even mild acids like lemon juice.

Cleaning
Warm, soapy water is always safe. Ultrasonic and steam cleaners are usually safe for untreated, heat-treated, and lattice diffusion treated stones. Fracture-filled, cavity-filled, or dyed material should only be cleaned with a damp cloth.

Treatment and durability considerations
Untreated and even heat-treated sapphire is very durable.

Stones that have undergone lattice diffusion treatment have varying degrees of treated-colour penetration. In some stones, the treated colour penetrates the entire stone, while in others there’s very shallow penetration. Surface damage or re-cutting can remove the colour from lattice diffusion treated stones with shallow colour penetration.

Fracture-filled stones have surface-reaching fractures filled primarily with high-lead content glass. The filler can be damaged through contact with a variety of chemicals. Even relatively mild substances like lemon juice can cause alteration of high-lead content glass.

The number of sapphires filled with high-lead content glass is much lower than the number of rubies. In some cases, the amount of glass filler in a sapphire can be quite extensive. Sapphires treated this way require greater care than untreated, heat-treated, or lattice diffusion treated sapphire.

Spinel
Spinel ranks 8 on the Mohs scale and has good toughness, making it a durable gem for jewellery.

Stability
High heat can cause some spinel colours to fade, but spinel is stable when exposed to light and chemicals.

Cleaning
While ultrasonic cleaners and steam cleaners are usually safe, certain inclusions like fractures could pose a potential problem. It’s always safe to clean spinel with warm soapy water.

Treatment concerns
Some spinel might be subjected to colour-altering heat treatment. The treatment is stable under normal wearing conditions. As with any transparent gemstone, spinel might be fracture-filled to improve its apparent clarity. This is rarely done to spinel, but if it is, the filler material can alter over time and affect the appearance of the stone. Any fracture-filled stone should only be cleaned with warm soapy water or a damp cloth.

Tanzanite
Zoisite (tanzanite) is 6 to 7 on the Mohs scale. It has fair to poor toughness and a property called cleavage, which is a tendency to break when struck. While tanzanite is not as hard or as tough as a stone like sapphire, it still can be worn in all types of jewellery with proper precautions against rough wearing or hard blows.

Stability
Tanzanite is stable under normal wearing conditions, which means it’s resistant to the effects of heat, light, and common chemicals. Even so, tanzanite should not be subjected to very high temperatures or sudden temperature changes. It can be attacked by exposure to hydrochloric and hydrofluoric acids.

Cleaning
Warm, soapy water is always safe. Ultrasonic and steam cleaners are never recommended for tanzanite

Treatment and durability considerations
Tanzanite is routinely heat-treated to modify its colour from a predominately brownish appearance to a predominately blue to purple appearance. The treatment is stable with no additional durability concerns. There have been instances where tanzanite was coated to modify the colour. Although rarely done, the coatings are not permanent and can be scratched or damaged through normal wear. Like other transparent gemstones, tanzanite can be fracture filled to improve apparent clarity, although this is an uncommon tanzanite treatment method. The materials used to fill fractures in gemstones can alter in appearance over time.

Topaz
Because of its basal cleavage, topaz requires special care during cutting, polishing, and mounting. It’s not very tough, so a hard blow might split it, and extreme pressure or sharp temperature changes might cause it to break. To reduce the risk of breakage, cutters orient faceted topaz so the cleavage direction is at about a 15-degree angle to the table. Jewellers prefer to set valuable topaz gems in protective mountings. Or they use topaz in pieces that aren’t exposed to too much wear, like pendants and pins. Topaz rates 8 on the Mohs hardness scale. Its toughness rating is poor due to its cleavage

Stability
High heat or sudden temperature changes can cause breaks in topaz. The gem’s colour is generally stable to light, but prolonged exposure to heat or sunlight might cause fading in yellow-to-brown, reddish brown, or dark brown topaz. Topaz is affected only very slightly by chemicals.

Cleaning
It’s important to avoid steam or ultrasound for cleaning topaz: Warm, soapy water works best.

Treatment and durability considerations
Besides irradiation and heat treatment, there is another treatment that involves coating colourless topaz with a microscopic layer of metallic oxide compound. The coating is not very durable. It can resist daily wear and tear, but abrasive cleaners or buffing wheels would remove it. It’s safe to use a mild soap solution. Despite these care considerations, fine topaz is a beautiful gem that offers a range of warm, attractive colours in large sizes that are often free of visible inclusions or flaws.

Tourmaline
Tourmaline ranks 7 to 7.5 on the Mohs scale. Its toughness is considered Fair.

Stability
Tourmaline is generally stable to light and isn’t affected by exposure to chemicals, but heat can damage a tourmaline. High heat can alter the colour, and sudden temperature change (thermal shock) can cause fracturing. Some tourmalines might have been treated to improve their colour. The two most important tourmaline treatments are heating and irradiation.

Turquoise
Turquoise ranks 5 to 6 on the Mohs scale and its toughness is generally Fair to Good.

Stability
Turquoise is generally stable to light, but high heat can cause discoloration and surface damage. It dissolves slowly in hydrochloric acid, and it can be discoloured by chemicals, cosmetics, and even skin oils or perspiration.

Cleaning
It’s safe to clean turquoise jewellery with warm, soapy water, but it should never be cleaned with steam or ultrasonic cleaners. Some turquoise is treated to improve its surface appearance. Heat or solvents can damage the treated surfaces.

Zircon
Zircon is rated 6 to 7.5 on the Mohs scale. It has fair to good toughness. While zircon is not as hard or tough as a stone like sapphire, it can still be worn in all types of jewellery as long as proper care is given to avoid rough wearing or hard blows. Also, zircon can be brittle, so it should be worn and stored in a manner that avoids hard rubbing against other materials.

Stability
Zircon is generally stable when exposed to light, but some heat-treated stones might revert back to their original colours (usually light brown) after prolonged exposure to bright light. Heat can alter the colour of some zircon. Zircon is stable when exposed to chemicals.

Cleaning
Warm, soapy water is recommended for cleaning zircon. Ultrasonic and steam cleaners are not recommended for cleaning this gem.