Gem Buying Guide: Gemstone Quality

Do you know the most important qualities in rare coloured gemstones?

Shopping for your perfect gem is more than finding the jewel that speaks to you. Use the information below as a general guide when shopping for coloured gemstones.

Determining a Gemstone's Hardness

Hardness is one of the most important factors in determining a gemstone’s durability.

It’s commonly defined as a gemstone’s resistance to scratches and abrasions.

The Mohs' Scale of Hardness which you see below ranks gem and mineral hardness on a scale of 1 (least hard – talc) to 10 (hardest of all – diamond).

Ideally, gemstones in jewellery should be 7 or higher.

This applies especially to rings since they will have to absorb more blows than earrings or necklaces.

Stones with a hardness below 7 can still be used in gorgeous rings, but should not be worn daily - think of these special pieces as occasion jewellery, such as a cocktail ring. Stones with a lower hardness level also work wonderfully but try to use them in earrings or necklaces instead!

Mohs Hardness Scale from

Mohs' Scale of Mineral Hardness from

Find the hardness of some other fine quality gemstones here:

Sapphire/ruby: 9
Spinel: 8
Morganite: 7.5-8
Malaya Garnet: 7-7.5
Tsavorite Garnet: 7-7.5
Paraiba Tourmaline: 7-7.5
Tanzanite: 6.5-7


See how these gemstones compare with other - generally considered lower quality - options in the marketplace:

Blue Topaz: 8
Amethyst: 7
Citrine: 7 (same family as amethyst)
Rose Quartz:  7 (same family as amethyst)


You may wonder why I put blue topaz in the lower quality option while it has hardness 8!

It's because this gemstone has other characteristics that make it less durable besides its hardness which is indeed quite alright.

It has what is called 'perfect cleavage' which means the stone can split perfectly when hit on its atomic weak points (like a diamond btw!).

Besides that, blue topaz undergoes quite some heavy treatments to turn it into its well-known blue shade.

For those reasons, it's not really considered a 'fine gemstone'.

FUN FACT: Diamonds also have weak spots to be aware of; while they are the hardest gemstone, they are definitely not the toughest stone out there!

“Toughness” is another factor that determines a gem’s durability, and indicates its ability to resist breakage or chipping from falls or impacts.

A diamond’s toughness is only
fair to good because they have a ‘cleavage plane’ and are therefore more fragile in some orientations than others.

And so, an accidental blow or fall on a stone floor can split a diamond if it happens to hit on the diamond’s atomic weak points. So beware!

Understanding the Refractive Index

In short, what you should try to remember of the refractive index or "R.I." is:

The higher the refractive index, the higher the brilliance of a gemstone.

Figuring out a stone's refractive index is typically the first test any gemologist will start with when handed a gemstone as it is a consistent metric for all gemstones. 

To dive a bit deeper into it: this number indicates the difference between the speed of light in a vacuum and the speed of light in the gemstone.

As light passes through a gemstone, it slows down since a gemstone is denser than air. This causes the path of light to change. Refraction is the bending of light as it passes in or out of a gem, and the angle of refraction in the stone determines its RI.

FUN FACT: Not every stone that has a high RI - including spinels, sapphires and rubies - will exhibit a fantastic brilliance under all circumstances.

The particular stone you’re looking at may, for instance, be heavily included on the inside which will hinder its natural brilliance.

So, you must judge every stone individually! 

Find the R.I. of several fine quality gemstones here:

Sapphire/ruby: 1.762 - 1.700
Spinel: 1.718
Malaya Garnet: approx. 1.714 - 1.888
Tsavorite Garnet: approx. 1.714 - 1.888
Tanzanite: 1.692 - 1.701
Paraiba Tourmaline: 1.624 - 1.644
Morganite: 1.577 - 1.583

Aquamarine and Tourmalines

A rock-sized aquamarine, neon mint and pink tourmaline in Eva's collection.

See how these gemstones stack up with other, generally less refractive options, in the marketplace:

Blue Topaz: 1.619-1.627
Amethyst: 1.544 - 1.553
1.544 - 1.553 (same family as amethyst)
Rose Quartz:
1.544 - 1.553 (same family as amethyst)

You probably have heard of these last 4 stones.

That is because they are plentiful in the earth which is why jewellers in the West typically use these stones in jewellery. 

On the other hand, the gemstones featured in the top list are much more rare in nature. And so, those gems have not been marketed on a large scale by jewellers because there simply are not enough of them to satisfy demand if they became ‘popular’.

These stones often remain the exclusive domain of the world's high jewellery houses, such as Chopard, Piaget, Christian Dior Haute Joaillerie, Louis Vuitton and Tiffany’s and can be found in dazzling, unique statement pieces.

Search for Specific Gravity (not just carat weight)

Specific gravity is another super useful tool to keep in mind.

It's a means of measuring the density of a gemstone.

It will help you understand the difference in size between different gemstone types even if those gemstones all have the same carat weight...

Did you know for instance that a 1 carat sapphire is smaller in size than a 1 carat diamond?!

To go into a bit more technical detail:

this measurement is achieved by judging the density of the gemstone relative to the density of water. And the specific gravity is expressed by a number that denotes how heavy the gemstone is in comparison to water of an equal volume.

For example, a gemstone may be two times heavier than an equal volume of water.

In that case, the specific gravity of the gemstone is simply 2.

The majority of gemstones exhibit a specific gravity that is between 2 and 4 times the equal volume of water.

Electric paraiba tourmaline and small tsavorite garnets from Eva's collection.

:  Don’t just say you’d like a "5 carat" or "4 carat" stone no matter what!

Now you know that "4 carat" doesn’t always mean the same thing among different gemstone types.

Look at a few stones in different carat weights and try to figure out the length x width x depth (in mm) that suits you and your hand best!

Do this by placing the stone on your hand which you hold flat, palm down, with you fingers closed, in the nook made by your middle and ring finger.

If you find a nice shape and size, remember this measurement (in mm) and tell your jeweller, so he / she knows what approximate size you like to have for your stone.

Then they will find the right carat size that will go with that measurement for any gemstone type.

You may end up buying a stone which is 3 carat or 4.5 carat because that particular size looks amazing on your hand.

This could mean that for some stones you may actually end up buying a smaller carat weight as the stone may turn out bigger because of this specific gravity: and as gemstones are priced by carat weight, this is a nice perk for your wallet!

The 4 Cs 

You've probably heard of the 4Cs in reference to diamonds... they are:

    1. Cut: refers to the facets, symmetry, dimensions and reflective qualities of a gem (this is different from the shape!)
    2. Clarityindicates the relative absence of inclusions (materials trapped inside the gem), fractures, and blemishes (surface imperfections) that affect a gem's appearance and structural integrity. 
    3. Carat: refers to a stone's weight, not its size. 
    4. Colour

These are good things to keep in mind when shopping for coloured gemstones as well. 

It may sound obvious but when shopping for coloured gemstones colour is the most important feature you need to look at!

(This is different from diamonds, where cut tends to be more important).

You'll want to make sure to look at three main things when judging the colour of your gemstones:

  • Hue: find a clear primary and secondary colour (for example: pinkish-red where red is the main body colour and pink its secondary hue) 
  • Saturation: more vivid and fully distributed colour without hints of grey or brown will make for the most valuable stones
  • Tone: the presence of blackness in the stone: medium tone is good for most stones  

Questions to Ask your Gem Dealer

"Can I see a few similar stones?"

  • It's important to view 2 or 3 additional stones that are similar in size, colour and shape to the one you have your eye on in order to compare colour. Don't forget this step!! Our brain is not wired to remember different colours very well. Only when you compare a few stones next to each other, is your eye able to distinguish among them and pick out the best colour. Also make sure to view them all in various lighting situations for the most accurate representation - in daylight, artificial light, at night, during the day etc.

"Is this stone natural or synthetic?"

  • Natural gems are entirely a product of nature and have not been altered by humans in any way other than ordinary cutting and polishing.
  • Synthetic gems have the same physical, chemical and optical properties of their natural counterparts but they are man-made - be aware of these and make sure they're available at a lower price.

If the stone is natural, ask: "has it been treated?"

  • Treated gems are natural but have been enhanced by undergoing some sort of treatment - heat treatments improve colour and clarity; this treatment is mostly acceptable and pretty common.

For more information on Real vs. Fake gemstones and how to tell them apart, explore this blog post.

"Can I get an independent gemological lab certificate?"

This is especially important for the more valuable or rare pieces but not for all gemstones. 

Hopefully this information helps you on your gemstone or bespoke jewellery journey.

And if you're ready to take a look at some rare gems via an in-person or virtual consultation, click on the link below. I'd love to help!

Until next time ✨

Eva xxx