By Tom and Kay Benham, Contributing Editors, Lapidary Journal Jewelry Artist
Beginning rockhounds and jewelry makers are often confused by the Mohs' Hardness Scale for rocks and gems. The Mohs Hardness Scale was set up in the early 1800s by mineralogist Friedrich Mohs, who selected 10 well-known minerals and numbered them in order of scratch hardness, such that a mineral will scratch all minerals having a lower hardness number. It served the purposes of comparison -- but because it is a non-linear scale, it does not provide a true indication of the relationship of the various hardnesses.
An examination of the chart at the right demonstrates that while corundum and diamond differ by only one number on the non-linear Mohs Scale, their hardness difference is four times greater on the linear Knoop Scale.
What does this mean to the beginner without any fancy equipment for measuring hardness? Well, by using simple items on hand, rockhounds can easily test for different hardness. Graphite, talc, and gypsum can all be scratched by a fingernail; calcite can be easily scratched with a copper coin; fluorite and apatite can be scratched with a pocket knife; a hardened steel file will scratch orthoclase; and quartz will easily scratch window glass.
You can assemble a simple kit using small samples of all the minerals listed on the Mohs Hardness Scale for testing the hardness of stones in the field. A diamond is really unnecessary, as it will scratch anything.