Gemstone Quality:
How To Choose the Perfect Gemstone

What are the most important qualities when sourcing rare-colored gemstones? Here, Eva shares her research and expertise in the field of gemology. For more information, or to arrange a first-hand consultation, please book an appointment.

1. Determining 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. Mohs Scale of Hardness (see table, below) ranks gem and mineral hardness on a scale of 1 (least hard – talc) to 10 (hardest of all – diamond).

Ideally, gemstones in jewelry 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 jewelry, such as a cocktail ring). Stones with a lower hardness level also work wonderfully in earrings or necklaces.

Mohs Scale of Hardness

Find the hardness of some 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 (purple): 7
Citrine (yellow or orange shades): 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 when 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 'perfect cleavage' which means the stone can split perfectly when hit on its atomic weak points (like a diamond). Besides that, blue topaz undergoes quite some heavy treatment to turn it into its well-known blue shade. For those reasons, it's not considered a 'fine gemstone'). 

Everyday Life Examples of Hardness

PLEASE NOTE: Diamonds also have weak spots to be aware of; while they are the hardest gemstone, they are not the toughest one 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. Diamonds’ toughness is only fair to good because they have a ‘cleavage plane’ and are therefore more fragile in some orientations than others. 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.

2. Understanding the Refractive Index:

The higher the refractive index, the higher the brilliance of a gemstone. This is typically the first test any gemologist will start with when handed a gemstone, as it is a consistent metric for all gemstones.

To go into technical detail: 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.

PLEASE NOTE: Not every stone that has a high RI - including spinels, sapphires and rubies - will exhibit a fantastic brilliance in all circumstances. The particular stone you’re looking at may be heavily included on the inside which will hinder its natural brilliance. So, every stone must be judged individually.

And if you took the quiz “Discover Your Gem” - all stone results from that quiz relate to ‘fine quality’ gemstones with a great hardness and a high RI.

Find the RI of several fine quality gemstones here:

Sapphire / Ruby:1.762 - 1.770
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 mint green and mint tourmalines

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

Blue Topaz: 1.619 - 1.627
Amethyst (purple): 1.544 - 1.553
Citrine (yellow or orange shades): 1.544 - 1.553 (same family as Amethyst)
Rose Quartz: 1.544 - 1.553 (same family as Amethyst)

The last 4 stones are plentiful in the earth, which is why jewelers in the West typically use these stones in jewelry. And why you’ll probably have heard of them.

The gemstones featured in my quiz are much more rare in nature. For that reason, those gems have not been marketed on a large scale by jewelers because there simply are not enough of them to satisfy demand if they became ‘popular’. That’s why these stones are often used in one-of-a-kind statement pieces, created by the best High Jewelry Houses in the world, such as Chopard, Piaget, Christian Dior Haute Joaillerie, Louis Vuitton and Tiffany’s.

3. Search for Specific Gravity (not just carat weight).

Specific Gravity is 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. For instance, a 1 carat sapphire is smaller in size than a 1 carat diamond.

To go into 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.

Gemstone Guide

EXPERT TIP:  Don’t just say you’d like a 5ct or 4ct stone no matter what! 4 carat doesn’t always mean the same thing among different gemstone types, which means you need to see what 4ct actually looks like on your hand. Compare sapphires, spinels, diamonds or tourmalines in order to figure out the length x width x depth (in mm) you prefer. 

Place 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. Remember this measurement 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 3ct or 4.5ct because that particular size looks amazing on your hand. This means that for some stones, you might actually end up buying a smaller carat weight (and as gemstones are priced by carat weight, this is a nice perk for your wallet)!

Want to learn more? Contact Eva to arrange a private consultation

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