hat these particulars of
grouping form a certain means of testing stones and of distinguishing
spurious from real. For if a stone is offered as a real gem (the true
stone being known to lie in the highest or cubic system), it follows
that should examination prove the stone to be in the sixth system, then,
no matter how coloured or cut, no matter how perfect the imitation, the
test of its crystalline structure stamps it readily as false beyond all
shadow of doubt--for as we have seen, no human means have as yet been
forthcoming by which the crystals can be changed in form, only in
arrangement, for a diamond crystal _is_ a diamond crystal, be it in a
large mass, like the brightest and largest gem so far discovered--the
great Cullinan diamond--or the tiniest grain of microscopic
diamond-dust, and so on with all precious stones. So that in future
references, to avoid repetition, these groups will be referred to as
group 1, 2, and so on, as detailed here.

CHAPTER IV.

PHYSICAL PROPERTIES.

B--CLEAVAGE.

By cleavage is meant the manner in which minerals separate or split off
with regularity. The difference between a break or fracture and a
"cleave," is that the former may be anywhere throughout the substance of
the broken body, with an extremely remote chance of another fracture
being identical in form, whereas in the latter, when a body is
"cleaved," the fractured part is more readily severed, and usually takes
a similar if not an actually identical form in the divided surface of
each piece severed. Thus we find a piece of wood may be "broken" or
"chopped" when fractured across the grain, no two fractured edges being
alike; but, strictly speaking, we only "cleave" wood when we "split" it
with the grain, or, in scientific language, along the line of cleavage,
and then we find many pieces with their divided surfaces identical. So
that when wood is "broken," or "chopped," we obtain pieces of any width
or thickness, with no manner of regularity of fracture, but when
"cleaved," we obtain strips which are often perfectly parallel, that is,
of equal thickness throughout their whole length, and of such uniformity
of surface that it is difficult or even impossible to distinguish one
strip from another. Advantage is taken of these lines of cleavage to
procure long and extremely thin even strips from trees of the willow
variety for such trades as basket-making.

The same effect is seen in house-coal, which may easily be