One of the most important and distinguishing qualities of a gem stone is the property of enduring, resisting wear,—in short, hardness. To test the hardness of precious stones that have not been cut or polished, the following scale of ten minerals has been devised by Moh, a German mineralogist:
No. 1. Talc. Very soft; is easily broken or scratched with the finger-nail.
No. 2. Rock-salt. Soft; scratched with difficulty with finger-nail; readily cut with a knife.
No. 3. Calcite. Low degree of hardness; not to be scratched with finger-nail; easily scratched with a knife.
No. 4. Fluor spar. Fairly hard; is slightly scratched by a knife, but easily attacked with a file.
No. 5. Apatite. Medium hardness; does not scratch glass, or only faintly; does not give out sparks against steel; easily attacked with a file.
No. 6. Felspar. Easily scratches glass; is attacked by a file, and gives some sparks against steel.
No. 7. Quartz. Quite hard; is only slightly attacked by file; gives sparks readily against steel.
No. 8. Topaz. Very hard; is not attacked by a file.
No. 9. Sapphire. Hardest of all minerals but the diamond; attacks all other minerals.