ewed, 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
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