From the formula given is made the material known as "strass," or
"paste," and stones made of it are mostly exhibited under and amongst
brilliant artificial lights. The mere fact that they are sold cheaply is
_prima facie_ proof that the stones are glass, for it is evident that a
diamond, the commercial value of which might be L50 or more, cannot be
purchased for a few shillings and be genuine. So long as this is
understood and the stone is sold for the few shillings, no harm is done;
but to offer it as a genuine stone and at the price of a genuine stone,
would amount to fraud, and be punishable accordingly. Some of these
"paste," or "white stones," as they are called in the trade, are cut and
polished exactly like a diamond, and with such success as occasionally
to deceive all but experts. Such imitations are costly, though, of
course, not approaching the value of the real stones; it being no
uncommon thing for valuable jewels to be duplicated in paste, whilst the
originals are kept in the strong room of a bank or safe-deposit.

In all cases, however, a hard file will abrade the surface of the false
stone. In chapter VII. we found that quartz is in the seventh degree of
hardness, and an ordinary file is but a shade harder than this, so that
almost all stones higher than No. 7 are unaffected by a file unless it
is used roughly, so as to break a sharp edge. In order to prepare
artificial diamonds and other stones for the file and various tests,
they are often what is called "converted" into "doublets" or "triplets."
These are made as follows: the body of the glass is of paste, and on the
"table" (see last chapter), and perhaps on the broader facets, there
will be placed a very thin slab of the real stone, attached by cement.
In the case of the diamond, the body is clear, but in the coloured
imitations the paste portion is made somewhat lighter in shade than the
real stone would be, the portion below the girdle being coloured
chemically, or mounted in a coloured backing. Such a stone will, of
course, stand most tests, for the parts usually tested are genuine.

A stone of this nature is called a "doublet," and it is evident that
when it is tested on the underside, it will prove too soft, therefore
the "triplet" has been introduced. This is exactly on the lines of the
doublet, except that the collet and perhaps the pavilions are covered
also, so that the girdle, which is generally encased by the mounting,
is the only surfa