Rocks are composed of minerals and crystals that can be tested for some of the common features that identify the “ingredients” of the rock. The basic tests that are done on rocks help to identify the components of the rock and to classify the rock in relation to other rocks. Just as botanists use the various characteristics of plants in order to identify and to classify them, geologists, mineralogists, and evaluators are highly interested in getting accurate and correct identification of the components and quality of the rock and its contents.
Because minerals have value and use, it is then important to know, for example, how much is there, what the quality is and whether the components will perform or react as expected. It can be a big difference between a perfectly clear diamond, a quartz crystal, and a man made zircon. Many a novice prospector or investor was left impoverished and disappointed during the California gold rush when it was pointed out to them that their find contained iron pyrite and not gold flakes.
In medicine, chemistry and pharmacology, it is critical that the mineral or crystal is exactly what it is claimed to be, or the results of using it may be disastrous or deadly.
There are six classic and basic tests that are done to identify and evaluate rocks. They are hardness, cleavage, chemical reaction, color, streak and luster.
Hardness: In 1822, Freidrich Mohs, a German geologist, developed a particular type of scratch test that is called the Mohs Scale. The Mohs Scale goes from 1 to 10, with talc being the softest at level 1 and diamond being the hardest at level 10. A great rule of thumb is the fingernail, the penny, the glass, the nail and the knife. If the fingernail produces a scratch, then the hardness is less that about 2.6 on the Mohs scale. The penny scratch means that the hardness is around 3. The knife or nail means a Mohs level of around 5 or 6. If the rock scratches glass, then it is estimated to be at 6 or greater in hardness.
Color: The minerals can have characteristic colors, but some minerals, such as diamonds and quartz can come in different colors. Color is then used as one of the ways in identifying minerals, but is not the most reliable when the same mineral can come in many colors or when a rock can present many colors to the naked eye.
Streak: One of the most common tools for testing mineral color is to leave a streak on unglazed porcelain tile. The streak test is based on the concept that, even though a mineral may have several colors that are visible to the eye, they will always leave a streak of a singular color that may not be the same as the rock that is being held.
Luster identifies how the mineral interacts with and reflects or absorbs light. Metallic and non-metallic are the two main classifications. Metallic luster will be very shiny and reflective, as with chrome and gold. Non metallic luster has many looks, the most common being classified as greasy or oily, glassy, waxy like a crayon, earthy or dull like dirt.
Cleavage identifies whether the rock breaks along planar fractures or breaks in irregular ways. The flatter , more regular, and cleaner the break, the better the cleavage rating. Shattering with no real pattern gets a poor cleavage rating.
Chemical reaction tests involve a weak acid, to specifically test for Calcium Carbonate (Co3), which will always fizz when exposed to acid.
“Rock Physical Property Tests”, Scienceviews.com
