ROCK HARDNESS CLASSIFICATION

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HARDNESS SCALE

The hardness of a stone is one of the properties that contribute to identification. Hardness is also an attribute which is important to be aware of, because it may determine what a stone may be used for (jewelry, carving, faceting, handling, storage, etc.) You may want to familiarize yourself with the Mohs Scale of Hardness. This will help you understand the relationships between stones according to their hardness.

The Mohs Scale of Hardness consists of 10 classifications, 1 being the softest, and 10 being the hardest. The only mineral that is an exception to this is mercury, which is liquid. To give you a few reference points, the diamond is of course the hardest, rated 10. Your fingernail is a 2, a pocket knife is about a 5-6, and a piece of glass is a 6-7. Each classification will scratch the one preceding it. As you know, a diamond (10) will scratch glass (6-7). This technique is commonly used in the field for initial identification, and it is good to have samples of some of these stones with you in order to perform the test. Some minerals have varying hardness according to the direction you may scratch them in, but typically this is either hardly detectable or an exception to the rule.

The following, is a partial listing of stones and their hardness classification:

1	Talc Graphite	Can be scratched with a fingernail and by any stone rated 2+
2	Gypsum Bismuth Lepidolite Chlorite	Can be scratched with a fingernail and any stone rated 3+
3	Calcite Celestite Barite	Can be scratched with a knife and any stone rated 4+
4	Flourite Malachite Platinum	Can be scratched with a knife and any stone rated 5+. Will scratch any stone rated 3-.
5	Apatite Dioptase	Can be scratched with a knife and any stone rated 6+. Will scratch any stone rated 4-
6	Feldspar Pyrite Amazonite Hematite	Can be scratched with a knife and any stone rated 7+. Will scratch any stone rated 5-.
7	Quartz Tourmaline	Will scratch glass and any stone rated 6-. Can be scratched by stones 8+.
8	Topaz Spinel	Will scratch glass and any stone rated 7-. Can be scratched by stones 9-10.
9	Corundum (ruby, sapphire)	Will scratch glass and any stone rated 8-. Can be scratched by diamond.
10	Diamond	Will scratch glass and all stones 1-9

Here are the hardness ratings for some metals:

Silver........2 1/2 - 3
Gold.........2 1/2 - 3
Copper.....2 1/2 - 3
Platinum...4 - 4 1/2
Iron..........4 - 5

HARDNESS SCALE SET
#KD-5 Includes 7 different rocks of varying hardness, and a chart of the Moh's Scale of Hardness. $12

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