In order to classify rock specimens scientists must conduct a series of tests in order to identify them. This testing is essential because so many rocks and minerals look so much like one another. This article will provide a brief explanation of the following scientific tests for rocks and how you can conduct them yourself on any rocks you find and want to add to your collection.
- colour test
- hardness test
- streak test
- lustre test
- lime test
- crystal structure
- cleavage test
There are wide variations in the colours of rocks. By using a good book about rocks and minerals you can compare the rocks you have in your collection to those in the pictures in the book to help you identify them. It is important to remember, though, that small amounts of impurities or combinations of other minerals can cause different color variations.
The Mohs Scale of Hardness is the test used to identify types of rocks. The scale goes from 1 to 10, with 1 being the softest and 10 being the hardest. In order to do this test you do need to have a sample of each type of mineral. Scratch the rock you are testing with the test specimen or several test specimens until you find one that scratches it. The scale of rock hardness on the Mohs Scale is:
To conduct this test on your rock specimens, you need to have a piece of unglazed porcelain or tile. Rub the edges of the specimen on the porcelain and examine the color that the streak makes. This colour is a clue to the type of rock or mineral you are testing. Now you can compare rocks that have a similar streak and eliminate those that do not match.
There are seven questions you must use to help you determine the lustre of the rock specimen you are testing. These are:
- Does it have a metallic lustre?
- Is it dull like clay or chalk?
- Does it have the same lustre as a pearl?
- Is the lustre non-metallic?
- Does it have a silky lustre?
- Does it have a greasy lustre?
- Does it have a vitreous lustre?
When you pour vinegar on the rock specimen it will bubble if there is lime present.
Examine the shape of the rock specimen. Different crystals have different shapes. A quartz crystal, for example, has six sides with six pyramid shaped ends.
The cleavage test determines how easy it is to split the rock specimen. Hit the rock with a hammer and take note of the lines that spread along a breaking point or cleavage.