Learn how to tell the difference between calcite and quartz, and other crystals.
Calcite is an underrated mineral which comes in multiple crystalline forms. Since there are so many potential formations for calcite, it can be hard to tell the difference between it and other minerals, especially if you are trying to judge a calcite based on the shape of its crystals.
Geologists know this, you see, and long since developed different testing methods to ascertain exactly which rock specimens they have on their hands. Learning these tricks can unlock the whole world of minerals for you.
Since we find the majority of our crystals ourselves here at The Stone circle, we have a bit of experience in telling the difference between common types of Scottish crystal. One of these is calcite, so allow us to talk you through testing whether or not your mineral is calcite.
How to Identify a Calcite Crystal/Chunk?
Calcite is another abundant mineral which is found worldwide. It forms in the ground, usually to the slow dripping and leaking of mineral-rich surface water over time. If you find creamy or white coloured crystals forming on another rock type, you may have calcite on your hands.
Use the below methods to identify a calcite crystal.
How to Test Calcite for Hardness
Calcite is a pretty soft mineral. When we test for mineral hardness we use a range called the Mohs Hardness Scale, which you may have come across before if you collect crystals. The Mohs hardness scale ranges from talc – the softest rock – up to diamonds, which were the hardest known mineral when the scale was devised. While talc scores just 1 point, diamond scores ten.
It’s worth mentioning that harder minerals have been found since, many of them man-made. For example, Forbes mention 6 minerals harder than diamonds in this article, if you have the interest to read more.
Calcite is a 3 on the hardness scale, which is not very hard at all and can make it crumbly. You can’t break it in your hands like you can talc and gypsum, but with enough force, a dropped crystal will break.
To test if your crystal is calcite, a stainless steel knife will help you. Steel has a hardness of around 5.5, meaning that a knife should leave a scratch mark on your crystal.
A quartz crystal, with a hardness of 7, should scratch your calcite.
The Streak Test
This is a reasonably easy test to see which crystal you have. A calcite streak is always white, regardless of what colour that calcite is.
To perform a streak test, you must scratch the mineral on a streak plate for an official result. However, if you use a piece of black slate, or a piece of unglazed porcelain, then you should get the same effect.
If you do not have unglazed porcelain at home, you can buy these on Amazon. Remember that we are affiliates and can earn a small commission through these links. You will find a good example of streak testing plates here.
The scratch will leave a white line. Anything other than white and your mineral is not calcite.
Examine the Lustre
The lustre of a mineral refers to the way it affects light when it hits it. Quartz might be described as glassy, while diamonds are excellent when polished and greasy when rough, while red jasper has a dull lustre unless you polish it.
Calcite has a lustre that ranges from dull to vitreous. Vitreous is the fancy word for ‘glassy.’ The more porous the calcite is, the duller it tends to be. If it looks pearly rather than glassy, you may have marble on your hands instead.
The Vinegar Test
The vinegar test is by far the fastest and simplest way to see if you crystal is calcite or not. Calcium is the mineral behind calcite, which is a calcium carbonate. The one thing that most calcium carbonates have in common is their reaction to acid. Since vinegar is an acid you likely have in your cupboard already, this is an easy-access science experiment.
How to perform the vinegar test on calcite?
Follow these steps to identify calcite using white vinegar:
1. Clean and dry your mineral to avoid contaminants.
2. Place your mineral in a shallow bowl.
3. Add the vinegar to your mineral, making sure you can clearly see any reactions it might have.
4. Calcite produces a mini amount of white bubbles, almost like a froth.
5. If there are no bubbles, try testing for quartz. The two can appear similar.
6. If the bubbles appear, you have identified calcite. Bubbles may also appear in limestone, but that’s a blog for another day.
Specific Gravity Testing
Calcite has a specific gravity of 2.71. Let’s break that sentence down until it makes sense.
The specific gravity relates to an equation. Each mineral has a different number assigned to their specific gravity, to help as another identification tool. It’s a bit maths-y, but if you are logically minded then this could be your go-to tool for rock testing… we are not maths-y.
The specific gravity of calcite is worked out by dividing the weight of the gem in air by the loss of weight in water. To do this, you weigh a set volume of water, say 100ml. You then place the gem into the water to see how much it rises by. By measuring the weight of the displaced water once the gemstone is removed again, you can perform the equation to figure out the precise number.
Honestly? I’m pretty sure I got that really muddled up so this page explains it better than me. Also, don’t blame us. It’s Archimedes’ fault.
Need Help with Mineral Identifications?
Asides from looking at photos of raw rocks online, you can use rock identification techniques to work out which rocks and minerals you have found. Rockhound Resource have a good list of identification methods, but you can also send minerals you fail to ID yourself to your local university.
Before you head to the nearest professor, seek help through one of the many Facebook pages which will identify rocks for you. Usually these are packed with geologists and rockhounds who can give you answers.
Hint
It’s probably not a meteorite. The saying goes that it may, in fact, be a meteowrong.
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