Hardness is perhaps one of the most important features in a stone, especially those of the "gem" series, for no matter how colour, lustre, general beauty and even rarity may entitle a stone to the designation "precious," unless it possesses great hardness it cannot be used as a gem or jewel. Consequently, the hardness of jewels is a matter of no small importance, and by dint of indefatigable research, in tests and comparison, all known precious stones have been classified in various scales or degrees of hardness. The most popular and reliable table is that of Mohs, in which he takes talc as the softest of the rarer minerals and classes this as No. 1; from that he goes by gradual steps to the diamond, the hardest of the stones, which he calls No. 10, and between these two all other gems are placed. Here is given a complete list of Mohs's arrangement of stones, according to their hardness, beginning at No. 1, thus:-- Talc 1 Rock salt 2 Amber 2-1/2 Calcite 3 Malachite 3-1/2 Jet 3-1/2 Fluorspar 4 Apatite 5 Dioptase 5 Kyanite (various) 5-7 Haueynite 5-1/2 Haematite 5-1/2 Lapis lazuli 5-1/2 Moldavite (various) 5-1/2-6-1/2 Rhodonite 5-1/2-6-1/2 Obsidian 5-1/2 Sphene 5-1/2 Opal (various) 5-1/2-6-1/2 Nephrite 5-3/4 Chrysolite 6-7 Felspar 6 Adularia 6 Amazon stone 6 Diopside 6 Iron pyrites 6 Labradorite 6 Turquoise 6 Spodumene 6-1/2-7 The Chalcedony group which embraces the Agate, Carnelian, etc. 6-1/2 Demantoid 6-1/2 Epidote 6-1/2