Searching out your own gemstones can be fun as well as profitable. And if you’re looking for a centerpiece for a special gift, there’s nothing better than a gemstone you’ve found yourself.
But how do you spot what could be a beautiful jewel before it’s been cut and polished? The good news is, it isn’t as hard as you might think. We’re going to take you through how to identify gemstones in the rough. And when we’ve finished, you’ll be ready for your very own treasure hunt.
So step this way to find out more!
1. Conduct a streak test
One of the most obvious characteristics used to identify a gemstone is its color. But when the stone is in its natural state, it can be hard to see exactly what that is. Dust and dirt can obscure it. And the color at the surface isn’t always the same as at the heart of the stone.
That’s where the streak test comes in. This uses a ceramic tile to scrape off a thin layer of the stone. The streak it leaves behind will give you a much clearer indication of the stone’s true color.
You can purchase a special streak plate online or at a hobby store. Or you can simply use a ceramic tile from a hardware store. Just make sure there’s no glaze, as that can interfere with the results.
To carry out the test, take a sample of the rock in your dominant hand (that’s the one you write with). Place the streak plate on a firm surface and hold it steady with your other hand. Then scrape the rock firmly across the plate.
You should find that it leaves behind a powder trail. It’s the color of this powder that will help you to identify the gemstone.
2. Dos and don’ts of the streak test
You need the uncut stone to leave behind a trail of powder, not splinters. To do this, you may need to apply quite a lot of pressure.
Don’t be afraid to press down hard. Some minerals are very hard, and you won’t get a powder streak by passing them lightly back and forth. (And some are too hard to get powder at all – more on that later.)
Scraping in more than one direction can also change the appearance of the powder streak. That will make it harder to identify what you’ve got.
Make sure that you clean the surface of the stone thoroughly before carrying out the test. Rinse it under running water, rubbing it with your fingers to remove dirt and debris. Then make sure it’s completely dry before testing. If it’s damp, it won’t leave behind enough powder for you to identify it.
Washing the stone will prevent any dirt from discoloring the powder. For the same reason, make sure you test a surface that’s been protected from the elements. Weathering from wind or rain can alter the composition of the surface layer, affecting the results.
Choose a pointy or ridged part of the stone to test. The smaller surface area will make it easier to get the powder.
And start the streak right up near the corner of the streak plate, moving the stone diagonally downwards. That will give you maximum room to get a good streak of powder.
Last but not least, it’s always a good idea to perform the test a second time to confirm the results. That will give you the confidence that you’ve identified the gemstone correctly.
3. Identifying the gemstone from the streak
Once you’ve got a good streak of powder, you can examine its color to identify the mineral. There are lots of charts online that will enable you to do this.
Note that the color will narrow down the possible gemstones, but many gemstones share the same hue. Some charts will also give you information on other properties that you can use to pinpoint your specimen.
A black powder streak, for example, could indicate graphite, galena, ilmenite or magnetite. If it leaves a mark on paper, it’s graphite. If it can be scratched by a penny, it’s galena. And if it’s magnetic and will scratch glass, it’s probably magnetite.
Other common colors for gemstone are greenish-blue for turquoise, pale blue for azurite and red for cinnabar.
Note that in some cases, the mineral will be harder than the streak plate. In that case, it won’t leave behind any powder and you’ll need to find other ways to identify it. Minerals that fall into this category include beryl, chrysoberyl and corundum (better known as sapphires or rubies).
4. Look at other properties of your gemstone
While a streak test is a good place to start, it may not be enough to be certain what kind of gemstone you have. Fortunately, there are lots of other clues you can look for.
One is to look at the way color changes through the rock. Some minerals show bands of color. Turquoise, for example, has a band of greenish-blue in gray rock.
5. Look to the light
Another thing to inspect is what’s known as the luster of the stone. This is the way the light reflects on its surface.
There are many different types of luster – metallic, sub-metallic and non-metallic. Non-metallic lusters are in turn sub-divided into vitreous (glass-like), dull, greasy, resinous, pearly, silky, waxy and adamantine.
Vitreous is the most common type of luster, appearing in about 70 per cent of all minerals. Examples include quartz, tourmaline and garnet.
The most reflective luster of all is adamantine. Drawing a boundary between vitreous and adamantine lusters isn’t an exact science. You may come across the term “sub-adamantine” used to describe specimens where it’s unclear whether they’re adamantine or vitreous.
A greasy luster, as the name suggests, indicates a stone that appears to be covered in a layer of oil. Jade, serpentine, vesuvianite and nephelite all have a greasy luster. More surprisingly, so do many uncut diamonds.
A resinous luster is also a feature in a number of gemstones used in jewelry. The name refers to the appearance of tree resin, and in most cases the stones are red, orange, brown or yellow. Amber, almandine garnet and sphalerite all have a resinous luster.
A silky luster comes from the way parallel crystals or fibers reflect light, similar to the sheen of silk. You’ll get a silky luster on gemstones including tourmaline and Tiger’s Eye. And some specimens of serpentine can also have this appearance.
Gemstones with a waxy luster have the appearance of – well, wax! Some forms of jade fall into this category, as well as opal and agate. And the chameleon-like serpentine can also have a waxy luster.
As you’ll see from this list, luster can vary between different specimens of the same mineral. So while it’s a useful aid to identifying your gemstone in the rough, it’s best used alongside other tests.
6. Look for imperfections
Different gemstones are characterised by different imperfections on their surfaces. In some cases, these are so distinctive they can allow you to identify the type of stone you’re looking at.
Pyrite, quartz and tourmaline, for example, are all covered in narrow grooves, known as striations. Each one occurs when two crystals faces form at the same time.
In some gemstones, these can develop into what are known as oscillatory striations. These are distinctive clusters of spikes – think of the crystals in Superman’s Fortress of Solitude. Spot these, and you’re almost certainly looking quartz, tourmaline or pyrite.
Another form of surface imperfection are “growth hillocks”. These are formed by spiral growths that push up through the crystal. They’re small lumps on an otherwise mostly flat surface. You may find them on gemstones like calcite and amethyst, and they can also appear on rubies.
7. Eyes On The Cleavage
Cleavage is the term used to describe the way crystalline gemstones fracture. “Perfect cleavage” means they will fracture cleanly along the planes of the crystal. Gemstones with no cleavage, on the other hand, will be very difficult to split.
Setting gemstones with perfect cleavage requires a lot of care. If they’re hit in the wrong place, they will fracture.
Some mineral charts include descriptions of the cleavage to help you identify the stone. The terms used to describe the different cleavage patterns are quite obscure, however. If you want to learn how to tell your conchoidal fracture from your cubic cleavage, be prepared for extra study!
Putting everything together
Identifying gemstones accurately means taking all these clues together. Take your time, and make sure you prepare your gemstones carefully before testing. Choose an unweathered surface for a streak test, and make sure it’s clean and dry before you start.
It’s a good idea to note down all your observations as you make them. Note the color of the streak test, and the luster of the crystals. Look for any surface details like grooves, pits or lumps. Check for bands of color, and look at the cleavage pattern.
Put all those pieces of information together and you’ll have everything you need to identify gemstones in the rough. We hope you enjoy your journey of discovery!