Look for one or more cracks in the stone. Sometimes so many that the
stone looks reticulated with a network of cracks, somewhat like the
pattern of tortoisehell although less regular.
Opals contain water in their structure; more or less water depending
on the source and the type of opal. Crazing occurs when the opal is
stressed; losing its water stresses the stone. Picture the cracks
which form as mud dries; the process is somewhat similar. The
conventional wisdom is that those opals which contain the most water
are the most susceptible to crazing. In opal like Honduras crystal for
example if you cut to grind away a crack usually a new one (or more)
will form more or less promptly as soon as the stone dries out again
Also, opals from volcanic matrix (eg basalt and rhyolite, such as
Honduras and Mexican) are held to be more susceptible to crazing than
opals from sedimentary formations (Australian). Opal diggers will
often keep the rough stones in a dry room, or out in the hot sun, for
a period of several months to a year; stones which have not crazed
under that stress are assumed to be safe for cutting & wear.
Some cracks may be may be shallow, others deeper. They can be
difficult to spot, particularly when you buy rough that’s been kept in
a jar of water or a water bath. Water hides cracks which are already
there and retards the formation of new ones. Oil is even worse. Never
buy opal that’s been kept in oil, unless you soak it in acetone first
to dissolve the oil out and then let it dry & check for cracks.
Cracks in a finished stone should be evident. When checking rough, a
good technique is to hold the (dry!) stone at the very edge of the
shade of one of those common “draftstman’s arm” type desk lamps. Let
the light fall into the stone but not into your eyes, in other words
look slightly down into the stone, at the same time turning the stone
to check from all directions. This helps show cracks clearly.