Natural Citrine or Heated Amethyst?
How to Tell Heated Amethyst from Real Citrine
People are often surprised to learn that natural citrine is relatively rare. Named after the French word 'citron' which means lemon, citrine is the yellow variety of the mineral quartz.
The vast majority of citrine sold around the world is heated amethyst. Brazil is the largest producer of 'fake' citrine, and ironically, it's also the world's largest producer of natural citrine.
Throughout history, humans have copied anything rare or highly sought after. With the advancement of technology, it's now possible to replicate virtually anything.
You may be surprised to learn that rocks and minerals have been imitated for thousands of years. The mineral turquoise is believed to be the first to have been copied. Fake turquoise has been found in ancient Egyptian tombs.
The ancient Greek philosopher Theophrastus noted that many stones change colour when heated. He went on to classify them according to the way they reacted to fire.
Ancient Roman author and naturalist Pliny the Elder also noted the effect that heat had on stones. In his encyclopaedia "Naturalis Historia" he wrote, "one gemstone can be changed into another." He also stated the colour of quartz could be changed into that of an emerald.
Heating Stones to Alter Colour
When amethyst is heated to 450°C (842°F), it turns yellow. Increase the temperature and it turns orange and then orange-brown. The final colour is determined by temperature and the length of time that it's heated. This is how 'fake' citrine is produced.
Exposure to heat changes the chemical composition of amethyst which leads to a gradual reduction in the amount of iron. Impurities of iron are the primary reason for the crystal's purple colour.
Heat is widely used to enhance or change the colour of many minerals. It can also be used to remove unwanted characteristics such as a wisp of colour.
Almost all blue topaz is heated. Although it does occur naturally, it's extremely rare. The colour of natural blue topaz bears little resemblance to the Sky, London and Swiss Blue gemstones produced through various heat treatments.
Most of the world's finest rubies and sapphires are heated. A ruby that's not been heated is extremely rare.
Aquamarine is also commonly heated to turn greener stones blue.
Despite heat treatments being an accepted enhancement, attitudes towards heated amethyst being sold as citrine seem to be shifting. 
Natural citrine is relatively rare and tends to be quite expensive. For this reason, heated amethyst has long been accepted as the next best thing.
Heating changes a stone's colour by interfering with its chemical composition. This process is similar to what happens in nature over millions of years.
Using heat to enhance or change the colour of rocks and minerals is not the same as dyeing them.
Agate and onyx are often dyed to make less interesting colours more desirable.
Fake Citrine: What to Look For
Although there are exceptions, it's usually quite easy to identify fake citrine. It's worth remembering the vast majority of commercial-grade citrine is heated amethyst.
One of the most obvious clues is the presence of white or colourless quartz. In a rough crystal, this will be visible towards the base.
A common characteristic of amethyst is colour zoning. This characteristic doesn't change when the stone is heated. A difference in the shade of colour will therefore be visible in fake citrine.
Colour is one of the easiest ways to identify heated amethyst. Heated amethyst will often have a rich almost burnt-orange colour. This can be more noticeable in lower-grade or poorly produced stones. 
Natural citrine can vary from light yellow to golden yellow to orange or can even be brownish yellow. Stones with subtle colours are far more abundant and less expensive than those with more saturated and uniform colour.
Another clue for identifying natural citrine is dichroism. Dichroism is a phenomenon whereby the colour of a crystal changes depending on the angle from which it absorbs light. Citrine is dichroic but amethyst is not.
The test is not always conclusive because some citrine is only slightly dichroic. Furthermore, smoky quartz and some other quartz varieties can also be used to produce fake citrine. Once heated they can also be dichroic.
So testing a stone for dichroism may confirm it's not heated amethyst, but there's no guarantee it's natural citrine. The test is not easy to carry out unless you know exactly what to look for.
Amethyst may not be dichroic but it is pleochroic. Minerals that exhibit pleochroism display different colours when viewed from different directions.
As a last resort to establish whether citrine is real, you could heat it, natural citrine fades when heated. The problem with this test is there's a 50/50 chance you'll ruin what may be genuine stone.
What's in a Name?
The main issue with heating amethyst to produce fake citrine is that some people, quite rightly, feel cheated. Heated amethyst is not natural citrine.
In the field of rocks, minerals and gemstones, heating and dyeing stones is relatively common.
Dyed agate or chalcedony is often sold as black onyx. The majority of coloured or banded 'onyx' is calcite. Blue topaz is white or colourless topaz that's been irradiated and lemon quartz is produced by heating amethyst or pale citrine.
Smoky quartz can also be replicated by heating clear quartz or citrine. Even natural citrine is often heated to enhance the colour.
Many stones have trade names that can be incredibly misleading because they do not reflect what the material actually is.
I have often wondered whether it would be more appropriate to call fake citrine yellow quartz, but that presents a whole new set of challenges. Yellow quartz is basically natural citrine just as purple quartz is amethyst.
Since the 4th century BC, amethyst has been synonymous with naturally occurring violet-to-purple-coloured quartz. Therefore quartz crystals in any other colour cannot be labelled as amethyst.
A relatively new material discovered in Patagonia in Argentina has been named pink amethyst. The name is contentious and tends to be used by those whose interest in rocks and minerals is metaphysical.
Some argue it should be called pink quartz but that's also problematic.
Rose quartz is the pink variety of the mineral quartz but there's another type of rose quartz often referred to as pink quartz. This material is far more scarce and has a slightly different chemical composition.
Pink quartz fades when exposed to sunlight and also forms individual crystals.
The colour of rose quartz is stable and its crystal habit is massive. This means crystals form one large intergrown mass with no external shape or structure. Individual crystals in rose quartz have never been found.
Different types of quartz have different names because their chemical composition varies, often due to impurities. An impurity is not the same as an inclusion.
The material known as pink amethyst has a slightly different composition to amethyst, rose quartz and the variety known as pink quartz.
Prasiolite is a rare green variety of quartz, most commercial-grade prasiolite is heated amethyst. It's often marketed as green amethyst. Amethyst cannot be green.
Another thing to be aware of is citrine rarely occurs as a geode. The geodes being sold commercially are amethyst that's been heated often in industrial-sized ovens. The crystals tend to have a burnt-orange colour. 
What is Madeira Quartz?
I believe Madeira quartz is a trade name that has only recently been introduced. It's clearly been taken from Madeira citrine.
Madeira citrine is a trade name for natural citrine with a deep saturated colour. The finest grade stones are rich golden-reddish-brown. Because of the similarity in colour, the name comes from the Portuguese fortified wine from Madeira.
I have however recently seen the name being used to describe darker-coloured fake citrine.
Although some people shy away from fake citrine, its production serves a purpose. Heating amethyst to produce 'citrine' allows consumers to purchase a similar-looking stone at a more affordable price.
Heated amethyst should always be less expensive than natural citrine.
Buying fake citrine can also be a more sustainable option because of the rarity of the natural stone. When demand is high for a mineral there's a significant risk of it being over-mined. Ocean jasper is a perfect example.
Despite heated amethyst often being labelled as 'fake' citrine, it's important to remember that this crystal is natural amethyst. When well produced, it can be very nice.
Heated amethyst tumbled stones (citrine) can be exceptionally colourful and often exhibit subtle iridescence.
With regard to fine-grade gemstones, the skill and expertise required to cut and polish is the same whether it's heated amethyst or natural citrine. With both being varieties of quartz, their hardness is the same.
Calling Heated Amethyst Fake Citrine
Terminology is important when describing heated amethyst. Although often referred to as 'fake' or 'imitation' citrine, neither word is particularly appropriate.I've used them in this article because both are widely used. Retailers generally don't tend to describe heated amethyst in this way. This is because heated amethyst is widely accepted as an alternative to natural citrine.
It's worth remembering that most blue topaz is produced by heating clear or colourless topaz. It's never described as fake or imitation blue topaz.
Clear quartz that's been heated to produce smoky quartz is never called fake or imitation smoky quartz.
In my mind, fake or imitation citrine would be glass, resin or some other man-made material, not a natural stone that's been heated.
When a stone that's been altered or enhanced is offered for sale, a customer should always be made aware of what they're buying. There's nothing in law however that requires a retailer to do this.
The correct terminology, at least for the time being, for citrine that hasn't occurred naturally should be heated amethyst.
The term 'heat-treated citrine' is sometimes used but can be misleading because natural citrine is often heated to enhance its colour.
The term 'baked' or 'cooked' amethyst is widely used in North America.
The tumbled stones in our final two photos are natural citrine. The darker stones come from the Democratic Republic of Congo and the lighter stones from Zambia.
Article Photos
The heated amethyst tumbled stones and three rough 'citrine' crystals come from our collection.
The retailer holding a handful of "natural untreated rough fine grade citrine crystals" comes from a listing on a popular online marketplace.
The photo of the pink amethyst geodes is courtesy of Bob Harman (FMF Minerals Forum). The photo is clickable and redirects to the original image which is worth seeing.
The citrine geode is being sold by a UK-based online business.
The heated amethyst (citrine) cut gemstone is from our collection.
The natural citrine tumbled stones in our last two photos are from our collection.
