Hematite is a fairly common mineral collected by junior rockhounds. More than likely you can go dig for Hematite in your state or you’ll find it at your local rock shop or gem and mineral show. Now that we know most people have Hematite in their collection we need to make sure what we have really is Hematite. Naturally, you’ll need the tools and a “how-to guide” for identifying Hematite.
You’re in luck! I’m a graduate gemologist from GIA and I’ve put together a robust “how-to guide” on identifying rocks and minerals in your collection. I’ve done my best to lay it out in a way that mirrors the way GIA teaches gemologists about identifying stones.
How to Identify Hematite Through Testing
There are various ways to identify rocks, minerals, crystals, and gemstones but we will be using a method I learned while attending the Gemological Institute of America. If you’ve learned a unique way to identify gemstones then feel free to share it with us.
Let’s take a deeper look into how to identify Hematite like a pro.
The visual inspection starts with what form of Hematite you have. The questions below are fairly easy to answer but each type will have its own process for identifying them.
Is it a cabochon? If you’re dealing with a cabochon then it should have a medium to high polish with very little pitting on the surface. You’ll notice it’s metallic in color but if the underside of the cabochon is not polished then it will look dark grey – black and you might even see a hint of red-brown. Remember, Hematite is made up of mostly iron.
Is it faceted? If you have a faceted piece of Hematite then it should be opaque. Try shining a light through it, you shouldn’t see transparency in the stone. The edges of the facets should be sharp but you could get chipping or abrasions.
Is it a specimen? Hematite is found in different forms and you’ll get better at identifying these forms by looking at and inspecting this mineral over time. Here’s a list of characteristics Hematite displays when it’s a specimen.
- Red or brown exterior coating with dark grey undertones.
- You’ll be able to do the streak test, keep reading below if you have a rough piece with no commercial value.
- An iridescence effect on the outside surface. This will look like a rainbow of colors, almost like a thin coating.
- It can be found in crystal form but it’s rarer and you’re not likely to come across it very often. These crystals are typically on the smaller side and considered to be micro-specimens.
Is it tumbled? Very common to find tumbled Hematite. Again, metallic shine with very little pitting. Dark grey in color with a nice silver-looking polish.
Physical Properties of Hematite
Let’s take a look at the physical properties of hematite. Knowing what to look for will help you more easily identify what you’re looking at.
Color: Metallic Gray, Dull to Bright Red
Clarity / Transparency: Opaque
Luster: Metallic to Splendent
Fracture: Irregular/Uneven, Sub-Conchoidal
The Streak Test
This is a destructive test so you need to ensure that you’re allowed to damage the specimen or stone if you choose to use this method. Once you’ve developed robust knowledge in identifying rocks and minerals you won’t be using destructive tests.
A mineral streak test is when you scrape the stone against a harder surface to see what color remains. When dealing with Hematite, you’ll notice a red or reddish-brown streak.
Raw hematite can be streaked across a piece of white paper to expose its color, hematites are always red.
Tumbled specimens are tested by scraping a specimen across a piece of ungalvanized porcelain, typically known as a streak plate.
If you conduct a streak test with what you think is hematite but down see the red-colored streak, you might consider that the specimen could be magnetite.
Hematite is not magnetic, so it shouldn’t respond to common magnets. However, there are exceptions to the magnet test.
For instance, it’s entirely normal for hematite to contain a considerable amount of magnetite. Since iron ore minerals are formed similarly, the two types may have combined during their chemical formation process. When this occurs, the chemical composition of hematite changes, making the hematite weakly attracted to magnets.
As a result, it’s possible to mistake a hematite specimen for either pyrrhotite or magnetite. That said, this is why we should consider utilizing all of the identification marks before ruling anything out.
I don’t recommend actively testing the hardness of a stone because it’s destructive in nature and doesn’t really provide a definite answer to what type of stone it is. With that being said, Hematite has a hardness of 5 to 6 on the Mohs hardness scale.
Refractive Index Test
Determining the refractive index, or RI as it’s referred to by gemologists, for Hematite is fairly straightforward but you’ll need a specific piece of test equipment and the RI fluid to go with it. Before you place the stone on the refractometer you want to make sure you have a flat, somewhat polished, surface to take a reading from.
Hematite’s Refractive Index: nω = 3.150–3.220, nε = 2.870–2.940
Each gemstone has its own RI, so discovering a sample’s RI can help you figure out what sort of stone it actually is.
Step 1 – Place a small bead of RI fluid on the metal surface of the refractometer near the back of the crystal hemicylinder (the window on which the stone will sit).
Step 2 – Place the stone facet face down on the fluid dot and slide it toward the middle of the hemicylinder crystal using your fingers.
Step 3 – Look through the viewer lens without magnification. Continue looking until you see the outline of a bubble, then look at the bottom of this bubble. Take the reading from there, rounding the decimal to the nearest hundredth.
From time to time, you’ll run into the issue of not having a flat surface to work with. In this instance, you’ll need to leave the top of the refractometer open and hold the rounded stone with your hand. Hopefully, you’ll be able to pull a reading off of the gauge.
You won’t be using this test for Hematite but I wanted to include this test just in case you were considering it in your process.
Consider testing the birefringence, as well. Birefringence is related to RI. While doing the birefringence test, you will turn the gemstone on the refractometer six times throughout the observation period and note the changes.
Perform a standard RI test. Instead of keeping the stone still, gradually turn it 180 degrees, making each separate turn about 30 degrees. At each 30-degree mark, take a new RI reading.
Subtract the lowest reading from the highest to find the stone’s birefringence. Round it to the nearest thousandth.
Birefringence: δ = 0.280
Single or Double Refraction
You won’t be using this test for Hematite but I wanted to include this test just in case you were considering it in your process. For this test to be accurate and beneficial the stone needs to be transparent in nature. If the light won’t pass through the stone then there is no way to test for single or double refraction.
Check for single or double refraction. Use this test on translucent and transparent stones. You can determine whether the stone is only singly refractive (SR) or doubly refractive (DR) to help identify it. Some stones can also be classified as aggregate (AGG).
Turn on the light of a polariscope and place the stone face down on the lower glass lens (polarizer). Look through the top lens (analyzer), turning the top lens until the area around the stone looks darkest. This is your starting point.
Turn the analyzer 360 degrees and watch how the light around the stone changes.
If the stone appeared dark and stays dark, it is likely an SR. If the stone starts light and stays light, it is likely AGG. If the lightness or darkness of the stone changes, it is likely DR.
Checking The Diaphaneity
Diaphaneity refers to the mineral’s ability to transmit light. For instance, some minerals are transparent or translucent. When they’re thick, a small amount of distortion might occur, but light will pass through them relatively freely.
Hematite is translucent. However, its translucency depends on the form it has taken. If the hematite has an earthy form, there won’t be much light traveling through it. Still, if it happens to take on a crystalline structure, you should expect an opaque diaphaneity.
Finding The Specific Gravity
Every stone has its unique specific gravity, which helps us identify them. Specific gravity is one of the best properties to measure when identifying mineral specimens. Most minerals have a narrow range of specific gravity, so getting an accurate measurement can go a long way toward identification.
Specific gravity is a unitless number describing how heavy a mineral is compared to equal volumes of water. For example, if a mineral is three times as dense as water, it’ll have a specific gravity of three. This is useful because while two minerals might be the same size, they’ll each have a different specific gravity.
The larger the sample, the more precise the readings tend to be. Remember that this technique can only be used for single mineral or crystal masses. It will not work for minerals embedded in host rocks.
Hematite’s Specific Gravity: 4.9 – 5.3
As helpful as specific gravity is for identifying minerals, amateurs are usually constrained by the lack of necessary tools for the job. However, one way to work around this is to hold the specimen and note how heavy or heft it feels compared to what you might expect a specimen of that size to weigh.
If you want to determine the specific gravity of your stone like a pro then you’ll need to invest in a higher-end scale. This is the one gemologists use OHAUS Density Determination Kit
Identifying Rocks and Minerals Like a Pro
Hopefully, you feel confident in your practice to identify a piece of Hematite after reading and applying this guide. You’ll be using the visual part of this guide the most and you’ll get better as you interact with more gemstones. Before you know it, you’ll be identifying stones like a gemologist.
If you run into any issues or you get confused then feel free to reach out and I’ll do my best with assisting you in the identification process.