Gemstone Properties: Hardness, RI, SG and Durability Explained

What are gemstone properties?

Every gem species has a fixed set of physical and optical properties that come from its chemistry and crystal structure. Sapphire is aluminium oxide arranged in a tight trigonal lattice; emerald is a beryllium aluminium silicate with a more open structure. That difference, together with the fractures most emeralds contain, helps explain why a sapphire shrugs off knocks that would crack an emerald.

Gemologists group gemstone properties into three practical sets:

• Durability properties: hardness, toughness and stability. These tell you how a stone will wear in a ring, pendant or earring.
• Optical properties: refractive index, birefringence, dispersion, pleochroism and fluorescence. These decide brightness and fire, and they are the main tools for identification.
• Physical constants: specific gravity (density), cleavage and fracture. These help identify a stone and explain why some gems look bigger or smaller for their weight.

Colour, clarity and cut are what you see first when you browse coloured gemstones, but these underlying properties are what decide whether a stone suits the way you will wear it. Some gems also show special effects, such as the star in a star sapphire or the play-of-colour in opal; our guide to optical phenomena such as star and cat's-eye effects covers those separately. Any term here you have not met before is also in our jewellery glossary.

What is the Mohs hardness scale for gemstones?

Hardness is a gem's resistance to scratching. Jewellers use the Mohs scale, devised by the mineralogist Friedrich Mohs in 1812, which ranks ten minerals from talc (1) to diamond (10). Each mineral scratches the ones below it.

The scale is relative, not even. The jump from corundum (9) to diamond (10) is far bigger than from 8 to 9: on absolute tests diamond is several times harder than sapphire. So a 9 and a 10 are not almost the same.

The number that matters most in daily life is 7. Ordinary household dust and grit contain quartz, which is 7 on the scale. A gem softer than 7, such as opal (5.5–6.5), tanzanite (6–7) or pearl (2.5–4.5), will gradually pick up fine scratches and lose the sharpness of its facet edges when worn daily. A gem at 7.5 or above resists that dust far better. Our full gemstone hardness guide lists more than 30 stones, and the tanzanite page explains how tanzanite rings handle everyday wear.

What is the difference between gemstone hardness and toughness?

Toughness is resistance to chipping, cracking and breaking from a knock. It is a separate property from hardness, and a hard stone is not automatically a tough one. Diamond is the clearest example: it is the hardest natural material, yet it has cleavage (planes of weakness along which the crystal can split) in four directions, so a sharp blow at the wrong angle can chip it. Its toughness is rated good rather than excellent.

At the other end, nephrite jade is only about 6–6.5 on the Mohs scale but is one of the toughest gem materials known, because its interlocking fibres resist breaking. Ruby and sapphire combine high hardness (9) with excellent toughness and no true cleavage, which is why they are the classic choice for a coloured stone worn every day.

Topaz shows why you need both numbers. At 8 it is harder than emerald, but it has perfect cleavage in one direction and can split cleanly from a single knock. Morganite and aquamarine, both beryls at 7.5–8, are tougher than emerald because they usually have fewer fractures. Our pros and cons of morganite and the wider guide to durability for rings worn every day go deeper.

Which gemstone properties matter most for an everyday ring?

A ring takes far more knocks than a pendant or earrings: door handles, benchtops, gym equipment, gardening. For a ring worn daily, look at hardness, toughness and stability together. This table is the summary we use across the counter.

If a birth month or meaning is driving your choice, our page on choosing a birthstone engagement ring for durability suggests tougher alternatives in the same colour, and engagement rings with colour and meaning shows how people balance beauty against wear. Tourmaline sits in the middle of the table; see tourmaline engagement rings for colour and durability for when it works well.

What does the refractive index of a gemstone tell you?

Refractive index (RI) measures how much a gem slows and bends light as it enters. Light travels through diamond about 2.42 times slower than through air, so diamond's RI is 2.42. The higher the RI, the more light a well-cut stone can send back to your eye, which is why high-RI gems look bright and glassy-sharp, and low-RI gems look softer.

Two related properties matter too. Birefringence is the split of light into two rays in doubly refractive gems; strong birefringence makes the back facets look doubled under a loupe, as in zircon and peridot. Dispersion is the split of white light into spectral colours, which you see as fire.

Ruby and sapphire sit at 1.76–1.77, which gives a fine stone its lively brilliance; colour still matters more than brightness, as our guide to judging ruby colour, clarity and cut explains. For more on this stone, see our ruby gemstones page.

What is the specific gravity of gemstones and how does it affect size?

Specific gravity (SG) is a gem's density compared with water. Sapphire's SG of 4.00 means it weighs four times as much as the same volume of water. Because gems are sold by carat weight (1 carat = 0.2 grams), a dense stone looks smaller than a light one of the same weight.

This is why comparing coloured stones by carat alone can mislead you: a 2 carat emerald faces up noticeably larger than a 2 carat sapphire. Cutters also vary in depth, especially in sapphires, so always compare millimetre measurements. Our guide to carat versus size and density has full charts. SG is also an identification tool: gemologists weigh a loose stone in air and then in water to calculate it.

How do heat, chemicals and treatments affect gemstone stability and care?

Stability is how well a gem resists heat, sudden temperature change, light, chemicals and moisture. A stone can be hard and tough yet still be damaged by the wrong cleaning method.

• Opal contains water (roughly 3–10%). Heat or very dry conditions can cause crazing, a network of fine cracks. Opal doublets and triplets should not be soaked.
• Tanzanite can crack with sudden temperature change; avoid ultrasonic and steam cleaners.
• Emerald is usually treated with oil or resin to fill surface-reaching fractures. Ultrasonic cleaning and solvents can remove the filler and make fractures visible.
• Pearl is organic calcium carbonate and is etched by acids, perfume, hairspray and even sweat over time.
• Peridot is sensitive to acids and rapid temperature change.
• Some amethyst and kunzite can fade with long exposure to strong light or heat.

Treatments change stability too. Heating sapphire and ruby is common and permanent. Lead-glass-filled ruby, where fractures are filled with a high-lead glass, is far less durable: the glass can be damaged by a jeweller's torch during repairs and etched by household acids. Irradiated blue topaz is stable in normal wear. Treatments must be disclosed, and our page on treated and untreated gemstones explains each one and how it affects value. For cleaning, follow the care instructions for each stone: for most gems, warm water, mild soap and a soft brush is the safe default.

How do gemologists use gemstone properties to identify a stone?

Gem identification is property-matching. No single test is enough, so a gemologist builds a picture from several measurements and compares it with known values. The usual sequence:

• Refractometer: reads RI and birefringence to two or three decimal places. This alone separates most species.
• Polariscope: shows whether a stone is singly or doubly refractive, separating spinel or garnet from sapphire.
• Hydrostatic balance: measures SG for loose stones.
• Microscope: reveals inclusions, growth patterns and treatment evidence, often the most telling test of all.
• Spectroscope and UV lamp: show absorption lines and fluorescence, such as the chromium lines in ruby.

Natural and lab-created gems are the hardest case. A lab-created ruby, sapphire, spinel or emerald has essentially the same chemistry, hardness, RI and SG as the natural stone, so the standard tests largely match. The difference lies in inclusions (curved growth lines or gas bubbles in some synthetic corundum, for example) and in advanced laboratory analysis. Neither is fake; they are different products and should always be described accurately. Simulants are another matter: glass or cubic zirconia imitating a gem shows quite different properties. Our scientific gemstone identification guide walks through each instrument.

Laboratories go further when they report origin, analysing trace elements and inclusion suites; see how labs determine a gem's origin. Australia is a significant source of several gems: precious opal from Lightning Ridge (NSW), Coober Pedy (SA) and Queensland's boulder opal fields; sapphire from central Queensland and the New England region of NSW; South Sea pearls from the north-west coast; and the pink diamonds of the Argyle mine in Western Australia, which closed in 2020. Some of these, along with stones such as alexandrite and demantoid garnet, feature among rare collector gemstones.

How to check a gemstone's properties before you buy: step by step

1. Name the stone exactly. Confirm the species and whether it is natural, lab-created or a simulant. This sets every other property.
2. Match hardness and toughness to your wear. For a daily ring, aim for Mohs 7.5 or higher with good or excellent toughness, or accept extra care.
3. Ask about treatments. Find out whether the stone is heated, oiled, filled, diffused or irradiated, and how that affects stability and repairs.
4. Ask for a report on significant stones. For valuable rubies, sapphires, emeralds and opals, an independent laboratory report confirms identity and treatment.
5. Compare millimetres, not just carats. Use SG to understand why stones of equal weight differ in size.
6. Choose a setting that suits the stone. Softer or brittle gems benefit from a bezel, halo or recessed setting that shields edges and corners.
7. Plan the care routine. Check whether ultrasonic, steam and heat are safe for that species before the first clean.

Setting choice is where gemstone properties turn into practical decisions; our guide to settings that protect softer stones shows which styles suit each gem.