A German mineralogist, Friedrich Mohs in 1822 set up a scale of hardness for minerals, and mineralogists have used it since. The scale goes from one to ten with ten being the hardest. Don't confuse hardness with toughness. The Mohs scale mainly shows what mineral will scratch another. The minerals shown below are representative of the degree of hardness for the Mohs scale:
When tumbling, it is recommended to use stones of the approximate same hardness to get the best results per stone. Listed below are other minerals or gemstones and their approximate hardness:
2-3: howlite, 2-4: chrysocolla, 3-4: malachite, 4: fluorite, 5: obsidian, 5-6: rhodonite, opal, 6: turquoise, feldspars (plagioclases), 6-7: garnet, 7: amethyst, citrine, chalcedony, agate, turritella, jasper, onyx, bloodstone, chert, flint, agatized or silicified petrified wood, tigereye.Common tests for hardness, compliments of Diamond Pacific Tool Corp., are as follows: Hardness 1 & 2 may be scratched by a fingernail. Hardness 3 may be scratched by a copper coin. Hardness 4 & 5 may be scratched by a knife blade or window glass. Hardness 6 through 9 scratches a knife blade or window glass. Hardness 10 scratches all common materials.