Thus when the line of sight is altered, or the stone moved, never otherwise, the colours chase one another over the surface of the gem, and mostly in broad splashes; but in those gems possessing "play of colour," strictly speaking, whilst the stone itself remains perfectly still, and the sight is fixed unwaveringly upon it, the pulsations of the blood in the eyes, with the natural movements of the eyes and eyelids, even in a fixed, steady glance, are quite sufficient to create in the stone a display of sparks and splashes of beautiful fiery light and colour at every tremor.

The term "iridescence" is used when the display of colour is seen on the surface, rather than coming out of the stone itself. The cause of this is a natural, or in some cases an accidental, breaking of the surface of the stone into numerous cobweb-like cracks; these are often of microscopic fineness, only perceptible under moderately high powers. Nevertheless they are quite sufficient to interfere with and refract the light rays and to split them up prismatically. In some inferior stones this same effect is caused or obtained by the application of a gentle heat, immersion in chemicals, subjection to "X rays" and other strong electric influence, and in many other ways. As a result, the stone is very slightly expanded, and as the molecules separate, there appear on the surface thousands, perhaps millions, of microscopic fissures running at all angles, so that no matter from what position the stone may be viewed, a great number of these fissures are certain to split up the light into prismatic colours causing brilliant iridescence. Similar fissures may often be seen with the naked eye on glass, especially if scorched or cooled too rapidly (chilled), and on the surface of clear spar and mica, their effects being of extreme interest, from a colour point of view, at least.

CHAPTER VII.

PHYSICAL PROPERTIES.

E—Hardness.

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
Haüynite 5-1/2
Hæmatite 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
Idocrase 6-1/2
Garnets (see also "Red Garnets" below) 6-1/2-7-1/2
Axinite 6-3/4
Jadeite 6-3/4
Quartz, including Rock-crystal,Amethyst, Jasper, Chrysoprase Citrine, etc. 7
Jade 7
Dichorite (water sapphire) 7-7-1/2
Cordierite 7-1/4
Red Garnets (see also Garnets above) 7-1/4
Tourmaline 7-1/4
Andalusite 7-1/2
Euclase 7-1/2
Staurolite 7-1/2
Zircon 7-1/2
Emerald, Aquamarine, or Beryl 7-3/4
Phenakite 7-3/4
Spinel 8
Topaz 8
Chrysoberyl 8-1/2
The Corundum group embracing the Ruby,Sapphire, etc.9
Diamond10