Rock Picture of the Week
Half the fun of
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.
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 is an indicator, not an
guide, as many
minerals come in different colors. Some minerals come in almost every
color and some in
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.
and grade from opaque to transparent.
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.
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.
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.
Hardness is measured on a scale of one (soft) to ten (hard). This is the scale.
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.
Include a magnet in your kit to test if the mineral is attracted to it, especially if it is dark in color.
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
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
nothing. If you need a lot of accuracy, weigh the stocking first and
then subtract that weight.
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
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.