The Different Quartz Varieties & Treatments
Quartz is one of the most important gemstone families, not only because it is the second most abundant material on earth (behind feldspar) but also because it comes in so many different varieties.
And in this post, we will look at the Quartz family in more detail and look at the different varieties of these amazing gemstones. If one or two are missing, it is probably because I don’t have one to take a picture of (no stock images on my site!). Ready to learn more? Then let’s get started…
What Is Quartz?
Quartz is the name for a group of minerals that share the same chemical composition, SiO2 (Silicon Dioxide, also known as Silica) and similar physical properties.
But one of the things that makes Quartz so interesting is that there are two distinct categories that we use to categorise Quartz, which are:
Macrocrystalline vs Microcrystalline Quartz
Macrocrystalline Quartz is the term for Quartz that forms as larger individual crystals (macro means big or large) that can be seen with the naked eye and are often referred to as varieties of Quartz, for example, Amethyst var Quartz.
They often form in clusters but can form as individual crystals. Either way, individual crystals will form in a hexagonal shape (even though they are part of the trigonal crystal system), usually with a pointed termination at one end but crystals with points at both ends can also be found.
Microcrystalline Quartz is the term used when the crystal is made up of lots of microscopic Quartz crystals that may be visible under magnification.
Unlike the Macrocrystalline varieties that form as well-defined crystals, Microcrystalline Quartz varieties form as what are known as massives or aggregates that have no defined crystal shape or form.
The Chalcedony varieties of Quartz are classified as Cryptocrystalline, which is a sub-category of Microcrystalline, as the individual Quartz crystals are not visible under magnification.
For this post, I’m going to put the Chalcedony varieties under the broader Microcrystalline category but there are some other differences than just crystal size between these two types of Quartz:
| Macrocrystalline | Microcrystalline | |
|---|---|---|
| Hardness | 7 | 6 1/2 – 7 |
| Specific Gravity (SG) | 2.65 | 2.58 – 2.91 |
| Refractive Index (RI) | 1.544 – 1.553 | 1.530 – 1.540 |
| Birefringence | +0.009 | up to 0.004 |
Let’s take a look at some of the varieties that fall within these two categories:
Macrocrystalline Quartz Varieties
Amethyst
By far the most well-known variety of Quartz is Amethyst. With colours ranging from pale lavender to deep velvety purple, Amethyst is not only is it one of the most beautiful (the most in my opinion, as purple is my favourite colour), but it is also the most historically significant.
It was once considered a rare and valuable gemstone that was held in the same regard as Ruby, Sapphire and Emerald until the large deposits were found in Brazil, which caused its status to drop a bit.
Citrine
A very close relative of Amethyst is Citrine, the pale to dark golden yellow variety of Quartz.
How close of a relative? Very! As it is the same element (Iron), which causes the colour in both stones but how come they have different colours? Well, it all comes down to heat, the yellow colour found in Citrine occurs when a crystal is exposed to temperatures in excess of 470 °C.
This is why natural Citrine is actually very rare, the majority of Citrine that is on the market is Amethyst or Smoky Quartz that has been heat-treated to bring out that beautiful yellow colour.
Ametrine
Ametrine is, as the name suggests, part Amethyst and part Citrine (gemmologically speaking it is a bi-colour quartz) but as with Citrine, these are very rare in nature as it requires very specific conditions in order for this to happen and there is currently one main source of natural Ametrine, which is Bolivia.
Most of the Ametrine that is available is either partially heat-treated natural Amethyst but this doesn’t take away from the beauty and uniqueness of these stones and they are my favourite variety as I love bi-colour gemstones.
Smoky Quartz
Smoky Quartz is probably the most overlooked variety of Quartz, probably as brown isn’t the most desirable of colours but they are still beautiful in their own right.
Smoky Quartz is probably the most abundant of all the crystalline Quartz varieties as it is found in multiple places around the world and it can form in some very large crystals where the weight is measured in grams or kilograms!
This means that faceted gems or cabochons are very cheap, making it an ideal gem for those just getting started with jewellery making.
Rock Crystal (Clear Quartz)
Rock Crystal is Quartz in its purest form as it is just Silicon Dioxide and was once a popular Diamond simulant until the likes of Cubic Zirconia (CZ) were invented, with crystals and cabochons now being the most popular form to buy Rock Crystal.
As with Smoky Quartz, Rock Crystal can form in large crystals but clean, cuttable material is quite rare, with many crystals having inclusions of some kind but some of these are actually very desirable, with the three main ones being:
Rutliated Quartz
This is Rock Crystal with golden Rutile inclusions, the number of inclusions varies from crystal to crystal but they are commonly cut as cabochons and when moved, the light often dances off the Rutile inclusions. Unfortunately, I don’t currently have one to take a picture of.
Tourmalinated Quartz
Similar to Rutilated Quartz but instead of Rutile, the inclusions are Black Tourmaline rods and while probably not the best description, it looks like hair trapped within the stone but they are a very unique and beautiful stone.
Strawberry Quartz
Strawberry Quartz gets its name from Hematite inclusions within the stone that have a red colour (due to hematite being Iron) similar to that of a strawberry. Depending on the amount of Hematite, some stones can have a nearly completely red appearance.
Rose Quartz
Rose Quartz is the pink variety of Quartz but differs from the other crystalline varieties in terms of appearance.
This is because high-quality, facetable Rose Quartz is very rare, with the majority of the material found being translucent and usually has a lot of fissures, which is why it is more commonly fashioned as cabochons or used for ornamental pieces.
It is also rare to find well-formed Rose Quartz crystals that have a typical Quartz shape, as you find with Amethyst and Smoky Quartz, instead, most Rose Quartz crystals don’t have a distinct shape.
Prasiolite
One of the rarest varieties of Quartz is Prasiolite (often incorrectly called green Amethyst), which is green in colour and found very rarely in nature.
But as with Citrine, it is quite a readily available gemstone, this is because nearly all of the Prasiolite that is on the market is heat-treated Amethyst and you might be confused by this as isn’t most of the Citrine available heat-treated Amethyst?
Yes, but Amethyst from certain locations, such as some Amethyst from Brazil, turns green instead of yellow when it is heat-treated and this is probably where the name green Amethyst comes from.
Chalcedony Varieties
Cat’s Eye Quartz
We start with my favourite variety, which is Cat’s Eye Quartz. From a colour perspective, they aren’t that exciting as they can be white, gray, green or yellow but what makes them special is that they possess an optical phenomenon.
This phenomenon is called Chatoyancy, this is where light reflects off parallel inclusions or rods within a gemstone when the stone is rotated and in some stones, it is called cat’s eye as the effect look similar to that of a cat’s eye.
Carnelian
Carnelian is a Chalcedony variety that ranges from orange to brownish red in colour.
Mostly cut as cabochons, stones can be translucent to opaque in appearance and can either be a single colour (reddish-orange being the most desirable) or possess multiple colours, with the most common being orange or red and white banding.
Sard
Sard is very similar to Carnelian, in fact, there isn’t a clear defining line between the two but Sard is generally known as the reddish-brown to brown Chalcendony variety.
If the stone has banded layers with Onyx, it is known as Sardonyx.
Agate
Agates are a beautiful Chalcedony variety. Agates come in a variety of different colours and can be single colour or banded with multiple colours.
But and it is a big but, a lot of agates on sale have been dyed and this is especially the case with stones or slices that have very bold colours such as bright blue or neon pink. Natural Agate colours tend to be more subtle.
There are also many different varieties of Agates, with some of the more popular being:
Moss Agate
Moss Agate is an agate with a colourless, translucent Chalcedony with green, moss-like inclusions, hence the name. Some stones also have red inclusions due to the presence of iron as an inclusion.
Dendritic Agate
Dendritic Agate has the same colourless, translucent base as Moss Agate but different inclusions. The inclusions in Dendritic Agate often look like trees or fern leaves but they aren’t caused by organic inclusions, instead, these beautiful inclusions are caused by iron or manganese.
Dendritic inclusions can also be found in other Chalcedony varities, there are also Dendritic Agates that are sometimes called Scenic Agates, as the inclusions resemble a landscape.
Blue-lace Agate
Blue-lace Agate is probably the most desirable variety of banded Agate due to the beautiful banded layers of pastel blue and white.
Onyx
Onyx is the black variety of Chalcedony. Even though pure black Onyx is the most commonly seen, usually fashioned as cabochons or flats, it rarely forms like this and most of these pure black stones are treated.
Natural Onyx is usually banded with black and white bands but can also have bands of other colour Chalcedony such as brown, which is called Sardonyx because it has bands of Sard and Onyx.
Chrysoprase
Chrysoprase is one of the more valuable varieties of Chalcedony. The most desirable colour is apple green but it can be lighter or darker, with some darker stones having a more Emerald-like colour.
Jasper
There are many different varieties of Jasper and I’m not going to list them here, with some named according to their colour, such as red jasper and others named due to their appearance, such as dalmation jasper.
Bloodstone (Helioptrope)
Bloodstone is probably one of the most descriptive Chalcedony varieties as the green stone with spots of red (caused by iron) looks like blood has been splattered across the stone.
Not the most romantic of descriptions but it is pretty accurate and is a popular variety, which is often fashioned as cabochons or flats.
Aventurine Quartz
Aventurine Quartz is the most sparkly of the Chalcedony varieties, as inclusions, usually Mica within the stone, give it a metallic sparkle that is also known as Aventurescence.
The most common colour for Aventurine Quartz is green but it can be found in other colours, including blue, orange and yellow.
Tiger’s Eye
Tiger’s Eye is a yellow to golden-brown variety of Microcrystalline quartz that also displays Chatoyancy but it is slightly different to that seen in Cat’s Eye Quartz as it is not unusual to see multiple refleted lines in Tiger’s Eye, where as Cat’s Eye usually shows one distinct line.
There is also a stone known as Hawk’s Eye, which is basically the same stone (they form under the same process) but is mainly blue in colour but displays the same Chatoyancy that is seen in Tiger’s Eye.
Quartz Treatments
I have mentioned a few treatments in the list above but I thought I would go into this in a bit more detail as a lot of the Quartz on sale, including crystals, cabochons and faceted stones have been treated in some way.
The main reasons why Quartz is treated is to:
- Turn a more common variety into a rarer one
- Make a poor quality gemstone look more attractive
So let’s take a look at some of these treatments.
Heat Treatment
Heat treatment is the process where they heat the gem to a certain temperature in order to change the body colour of the stone.
This is commonly done to Amethyst and Smoky Quartz in order to turn it into Citrine, some Brazilian Amethyst will turn green and become Prasiolite.
It is a very difficult treatment to detect and it is assumed that the majority of Citrine and Prasiolite were originally Amethyst.
Crackled Quartz
Crackled Quartz is Rock Crystal that has been heated up and the dropped into water containing a coloured dye, this causes the stone to fracture and the dye gets pulled into any surface reaching fracture.
The result, depending on colour can look similar to other Quartz varieties at first glance, such as the two pictured that could resemble Amethyst and possibly Strawberry Quartz.
But when taking a closer look, you will see that colour is concentrated in the fractures within the stone (as you can see with the stones pictured) and when exposed to heat or chemicals, the dye can come out of the stone.
This is commonly done with tumblestones but can also be found in cabochons and faceted stones.
Dying
Dying is much more common in microcrystalline Quartz varieties as they can be porus due to minute gaps between the crystals.
And while the stone pictured is an extreme example with the neon pink dye, it isn’t unusual to find Agates, especially Agate slices in what are pretty unatural colours.
Sometimes the dying is more more subtle and done in a way to make lower quality material look like more desirable.
The easiest way to spot this is to take a closer look at the stone and just like with the Crackled Quartz, you will see colour concentrations in the stone, where as natural stones have the colour even spread across the stones as it is the crystals that are coloured, not the gaps between them.
Coating
Coating is more commonly found with Quartz crystals that with fashioned stones.
Many of the crystals are sold with fancy names as healing crystals, such as Aurora Quartz.
In reality, what they are is low quality Quartz crystals, coated in a material, such as Titanium and the result is a crystal that displays multiple colours when the light reflects of the coating.
I haven’t seen a coated cabochon yet but I’m sure they exist.
When it comes to faceted gems, coating most notable observed in what is often called ‘Mystic Quartz’ (similar to Mystic Topaz), this is where the pavilion of the stone has been coated with a material that produces a spectral colour effect when the stone is moved under light.
You can usually tell if a faceted stone has been coated as the coating wears off quickest on the facet edges and if the stone is loose, the lustre on the pavilion facets will be different to the crown facets.
I'm Paul Haywood FGA DGA, the owner and founder of Haywoods Gems, I'm a fully qualified Gemmologist and Diamond Grader from the Gemmological Association of Great Britain.
