Rock Picture of the Week

Field Testing

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Half the fun of rockhounding is identifying the specimens that you find. With the help of a field guide and a few simple tools, you can do that most of the time.
The first thing you need to do is learn the difference between a rock and a mineral. A rock is composed of more than one mineral, for example "Granite".
Granite is made up of Quartz, Feldspar, and usually some type of Mica. Looking at the rock you can see white or clear particles (Quartz), pink or gray particles (Feldspar), and dark spots (Mica).
Quartz, Feldspar, and the Micas are examples of minerals.

Each mineral has it's own set of characteristics, such as color, hardness, luster, and more. Each test that you make will narrow down the possible choices that the mineral can be.

A good rock hunting field guide is a big help. I grew up using "A Rock-hunter's Field Manual" by Fritzen, but I'm sure that there are other good books out there. Try to find one that shows the characteristics of the minerals.

Another option is to download a computer guide and database at Bob's Rock Shop.

There is also a pretty good online minerals database at www.mindat.org. Click on "Search Minerals by Properties" and plug in the data that you've gathered from these tests. 

Color
Luster
Streak
Cleavage
Fracture
Hardness
Magnetism
Specific Gravity
Crystals

 


Color

Color is an indicator, not an iron-clad guide, as many minerals come in different colors. Some minerals come in almost every color and some in just one.
Often the color defines the sub-group of the mineral. Examples are Ruby and Sapphire, both are varieties of Corundum.
Color is best determined by viewing a fresh surface, such as a new fracture, or by wetting. Avoid licking minerals, as some are very poisonous. It's best to carry some sort of wetting agent.

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Luster

Luster is the appearance of the mineral in reflected light. It can fit in more than one of the categories.

Metallic      Having the appearance of metal, such as silver, gold, steel, copper etc. Almost always opaque, where light will not shine through even thin edges.

Submetallic     Partial metallic sheen. A very subjective category that grades from metallic to nonmetallic. May be opaque or translucent on thin edges. Translucence is where light can be seen through thin edges, but you can't see through it.

Nonmetallic     Can be any  color and grade from opaque to transparent.
It may look;
Dull or earthy
Vitreous or glassy
Resinous or waxy
Adamantine or brilliant
Pearly
Greasy
Silky
And sometimes other terms.

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Streak

Streak is the color the mineral when powdered. The most common way to view this is by observing the color of the residue left when the mineral is rubbed against a piece of unglazed porcelain. It is often different from the observed color of the mineral. Very useful with opaque and metallic specimens.

I recommend putting a streak plate in your test kit. Porcelain tiles are common and you can get them from tile shops or home supply stores. Be careful with broken pieces, as they can be razor sharp. Dull the edges with a grinder or sand paper before using.

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Cleavage

Cleavage is when a mineral tends to separate along it's crystal faces. Usually it is in parallel leafs, often leaving a step-like appearance, sort of like tearing apart a deck of cards that is stuck together. A photo example is Gypsum. Diamonds are often shaped using cleavage lines.

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Fracture

Even if the mineral shows no cleaving, you can still get information from breaking off a piece with a hammer. The broken surface will show characteristics such as granular, smooth, jagged, concoidal (shell like), and others.

Also the mineral may be brittle, tough, malleable, flexible, etc.

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Hardness

Hardness is measured on a scale of one (soft) to ten (hard). This is the scale.

1 Talc
2 Gypsum
3 Calcite
4 Fluorite
5 Apatite
6 Feldspar
7 Quartz
8 Topaz
9 Corundum
10 Diamond

I recommend carrying some hardness testing items in your field kit. Some common items are: penny 3, knife 5, glass or file 6, and a piece of quartz at 7.

If Quartz scratches the mineral then it is softer than 7. If the mineral scratches Quartz then it is harder than 7.

For an example, if a file doesn't scratch your specimen, but Quartz does, then your specimen has a hardness somewhere between 6 and 7.

 

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Magnetism

Include a magnet in your kit to test if the mineral is attracted to it, especially if it is dark in color.

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Specific Gravity

Specifitc gravity is the weight of the mineral compared to the weight of an equal volume of water. This is a test that is very hard to do in the field without special equipment, but fairly easy at home. You need an accurate scale to get decent results. You can use a platform scale, where the specimen rests on top, or a suspension scale where it hangs below. You can use as many pieces of the specimen as you want as long as they are all the same mineral.

Using a suspension scale is the easiest way, but they are not as common. A helpful item is a nylon stocking. You can buy a pair of them in a plastic "egg" in most department stores for under $0.50 and they weigh practically nothing. If you need a lot of accuracy, weigh the stocking first and then subtract that weight.
Place your specimen(s) in the sock and suspend it from the scale. Record the specimen weight and lower it into a container of water until it is submerged.
Again record the weight and subtract this weight from the first. The difference is the water weight.
Divide the specimen weight by the water weight to get the specific gravity.   Clear as mud Huh?

Using a platform scale, weigh your specimen(s) and record it. Find a water container that will hold your specimen(s) and that you can catch any overflow water. Maybe a tea pot. Fill the pot with water until it overflows. Empty the catch container and carefully submerge your specimen, catching all the overflow water.
At this point you can weigh the water or measure it.
To weigh it place the catch container on the scale and record the weight. Dump the water out and weight the empty container. Subtract the empty container weight from the full weight to get the water weight. (wait, there's more)
If you measure the water, 1cup weighs 8oz, and in metric, 1cc (or 1ml) weighs 1g.
Again, divide the specimen weight by the water weight to get the specific gravity.

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Crystals

If your specimen has crystals it is probably the best way to determine what it is. It is a large and complicated field and there are others with more information than I have. The link below does a pretty good job of explaining what you need to know and more.

Introduction to Crystallography and Mineral Crystal Systems
by Mike and Darcy Howard

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