Categories of Rocks and Characteristics of Minerals

Geologic Time and Rock & Mineral Identification Guide Adapted from Home Science Tools (2008b)

Geologic Time (Radiometric Dating Techniques): Answer the questions below.

1. What unstable isotope would be best to refine the date of bones found in a cave hearth built by humans between 30,000 and 50,000 years ago?


2. We find samples of an igneous rock demonstrate it has been through 4 half-lives. The test element has a half-life of 150 million years. How old is the rock?


3. Argue for or against the following: A stone tool fashioned from a chunk of obsidian yields a date of 5,000,000 years old, therefore, the tool was made by a human 5,000,000 years ago.


4. You are trying to figure out the age of what is thought to be a very old fossil with a volcanic ash layer immediately above the fossil. We know the fossil is at least more than 250 million years old. Should we use carbon 14 to date the fossil, or uranium 238 to date the volcanic ash layer, and why?


5. If the parent isotope starts with 100 grams, but your samples yield only 12.5 grams of the parent isotope, how many half-lives have passed?

Rock & Mineral Identification

Minerals are naturally occurring, solid substances composed of chemical elements. This means that minerals, ranging from salt to rubies, are made up of compounds of elements that appear on the periodic table. Each type of mineral has a specific chemical composition and consistent physical properties. They are inorganic, not living or made up of living things. Minerals form a crystalline structure which gives rocks their "rough" texture.

Rocks are mixtures, or aggregates, of different minerals. Some rocks, like limestone, are composed mostly of one mineral, but the majority of rocks are made up of several major minerals. Rocks are divided into three categories based on how they are formed: igneous, sedimentary, and metamorphic.

Igneous rocks form when hot magma from beneath the Earth's surface cools rapidly. Igneous rocks that cool beneath the surface (perhaps by hitting underground air pockets) are called intrusive or plutonic igneous rocks. Granite is an example. When the magma cools on the Earth's surface by flowing from the mouth of a volcano as lava, the resulting rock is called extrusive or volcanic rock. Basalt and obsidian are common examples of volcanic igneous rocks.

Sedimentary rocks are formed by layers of sediment accumulating and being compressed together for extended periods of time. Most sedimentary rocks have layers, and they often contain fossils, as living material was buried in the sediment before it was compressed into rock. Common examples of these rocks are limestone, sandstone, and shale.

Metamorphic rocks are rocks that have been changed by high pressure or heat. The crystal structure is changed, the texture often becomes coarser, and sometimes new minerals are formed in the process. Metamorphic rocks are the most complex group of rocks. Schist, slate, and gneiss (pronounced like "nice") are common examples.

Identifying Rocks

Color. As a general rule, darker rocks are made of minerals with iron and magnesium, such as magnetite or biotite. Lighter-colored rocks may have lots of quartz, calcite gypsum, or halite in them.

Texture. Is it coarse-grained or glassy-smooth? Is it dense with very small particles? Are minerals visible to the naked eye? The texture of a rock depends on what is it made of. For example, igneous rocks go by basic crystal size, and sedimentary rocks will have a texture of clastic, chemical, or biogenic, depending on how they formed. Metamorphic rocks will be identified by whether they are foliated or not foliated.

Structure. Look for layers, which are often an indication of sedimentary rocks. Some volcanic igneous rock will have a sponge-like structure – pumice is an example of this. It is less dense than water, so it floats! Sedimentary rocks may have layers in them, but this is more common to shales. They can also have fossils, or banding.

Minerals. Look at individual grains with the magnifying lens and see if you can identify any of the composite minerals. With larger grains, you may be able to identify what they are since you are also learning about mineral identification this week.

Acid Test. Limestone contains a carbonate compound that dissolves in acid, producing bubbles. Test for bubbles with a few drops of vinegar.

Igneous Rock Identification

↓Texture/composition→        Felsic              Intermediate              Mafic

Phaneritic (coarse)      Granite                                    Diorite                         Gabbro

Aphanitic (fine)          Rhyolite                      Andesite                     Basalt

Glassy                                                Obsidian

Vesicular                                            Pumice

Sedimentary Rock Identification

↓Composition/Texture→       Clastic            Chemical                    Biogenic

Grain size >2mm (coarse)      Conglomerate              

Grain size 2 mm – 0.062 mm

(medium)        Sandstone                    

Grain size < 0.062 mm (fine) Shale                

Calcite                                    Limestone                               Fossiliferous Limestone

Calcite                                    Calcareous tufa         

Metamorphic Rock Identification

↓Composition/Texture→       Foliated          Nonfoliated

Quartz, feldspar, mica            Gneiss

Mica visible crystals           Mica schist        

Minerals not visible crystals   Slate   

Quartz                                                           Quartzite

Calcite                                                           Marble

Minerals Identification

Three tools are provided in your kit to aid you in identifying each mineral:

• Magnifying Lens:  This is one of the most important tools for a mineralogist, because identifying minerals involves close observation.


• Nail:  A nail is one of the many common items you can use to test the hardness of your specimens, along with a fingernail, penny, and piece of glass.


• Streak Plate:  A streak plate is used to determine the color of a mineral in powder form.

Many minerals can be identified using close observation and some simple tests. (Results are most consistent if you test on a freshly-broken surface of the mineral.) Try these steps on your specimens and see if you can identify each one using the characteristics provided in the online resources on mineral identification.

Luster. Luster refers to the way a mineral reflects light. Is it shiny like metal? Then its luster is called metallic. It could also be adamantine (brilliant, like a diamond) or vitreous (glassy, like quartz.) Other common terms to describe luster are dull, earthy, silky, greasy, or pearly. Transparency is another characteristic that is related to luster. If you can see through the specimen, it is transparent. If light can pass through, but it is not see-through, the mineral is translucent. Minerals that do not let light through are called opaque.

Color. Note the color of your specimen. This can be helpful for identifying metallic minerals, but many nonmetallic minerals have variable colors because of impurities. Quartz comes in many different colors and sapphires and rubies are different-colored varieties of the same mineral, corundum.

Streak. A streak test determines the color of a mineral in powder form. In some cases, especially for metallic minerals, the streak may be a different color than the lump form of the mineral. In these cases, streak can greatly aid identification. In general, streak is more useful in identifying dark-colored minerals than light-colored specimens. The most common way to do a streak test is to rub your sample across a ceramic plate. If the mineral has a hardness level less than the streak plate (7) it will leave a colored streak of powder. (Wash the streak plate with soap and water as necessary.)

Hardness. Mineral hardness is measured on the Mohs Hardness Scale. On each level of the scale a mineral can be scratched by something of the same or higher level, but nothing lower. The scale is made up of 10 minerals varying in hardness from 1 to 10. Number one is talc, because it is soft and very easy to scratch. Number 10 is the diamond, because it is the hardest natural substance and can only be scratched by another diamond.

1. 1.      Talc


2. 2.      Gypsum


3. 3.      Calcite


4. 4.      Fluorite


5. 5.      Apatite


6. 6.      Feldspar


7. 7.      Quartz


8. 8.      Topaz or Beryl


9. 9.      Corundum
10. 10.  Diamond

Number one is talc, because it is soft and very easy to scratch. Number ten is the diamond, because it is the hardest natural substance and can only be scratched by another diamond. You can test the hardness of your specimens using common materials like a nail, which has a hardness of about 5, or a streak plate with a hardness of 7. You can also try using a fingernail (2.5) a copper penny (3), or a steel file (6.5). Hold the specimen firmly and drag the nail across it. You will feel if it catched on the mineral or if it just slides off it without biting into it. Use your magnifying lens to look for a scratch. If your specimen can be scratched by the nail (5) but not by a copper penny (3), its hardness is between 3.5 and 4.5.

Other Tests

The above tests should allow you to identify the minerals in this kit, but there are other tests you could also perform on to help you identify unknown minerals using a field guide or web resource. Some of these tests are described below.

Cleavage. Cleavage refers to how a mineral breaks. If it tends to break in smooth, flat planes it has cleavage.  (If it breaks to form jagged edges only, it has fracture instead.) There are varying degrees of cleavage based on how clean the break is. If a mineral is transparent or translucent, you can often see cleavage planes with a magnifying lens, without having to break the mineral. Watch out! Sometimes crystal faces on a mineral look like cleavage planes, and vice versa.

Magnetism. Some minerals (like magnetite) are magnetic. Test your specimens to see if they are attracted to an iron nail or magnet.

Acid Test. Certain minerals, such as calcite, have carbonate compounds that dissolve in acid, producing bubbles. You can test this by roughing up a corner on your streak plate, then putting a few drops of vinegar on your specimen and watching for bubbles.

Fluorescence. You can try shining a black light on your specimens to test for fluorescence. Some minerals absorb ultraviolet light and emit visible light, making them glow in the dark with various colors.

Specific Gravity. Knowing a mineral's specific gravity can help with identification. Specific gravity is the weight of a mineral compared to the weight of an equal volume of water.

Original Resource

Home Science Tools, Ltd. (2008a). Mineral study kit.

To use the ID tables, be methodical. Start with luster, then the color of sample, then hardness, and so on.