somewhat
after the form and size of bunches of grapes ; hence, the name "
botryoidal" is often applied lo this variety. The last and commonest of
all the seven varieties of opal is somewhat after the shape of a kidney
(reniform), or other irregular shape, occasionally almost transparent,
but more often somewhat translucent, and very often opaque. This seventh class is called "meni-lite," being really an opaline form of quartz, originally found at Menilmontant, hence its name (Menil, and Greek lithos, stone). It is a curious blue on the exterior of the stone, brown inside.
History
records many magnificent and valuable opals, not the least of which was
that of Nonius, who declined to give it to Mark Antony, choosing exile
rather than part with so rare a jewel, which Pliny describes as being
existent in his day, and of a value which, in present English
computation, would exceed one hundred thousand pounds.
Many
other stones possess one or more properties of the opal, and are
therefore considered more or less opalescent. This " play of colour "
and " opalescence." must not be confused with " change of colour." The
two first appear mostly in spots and in brilliant points or flashes of
coloured light, or " fire " as it is termed. This fire is constantly on
the move, or "playing," whereas " change of colour," though not greatly
dissimilar, is when the fire merely travels over broader surfaces, each
colour remaining constant, such as when directly moving the stone, or
turning it, when the broad mass of coloured light slowly changes,
usually to its complementary. Thus in this class of stone, subject to "
change of colour," a
