Opal may be anything from crystal clear to completely opaque.
Depending on its transparency the term Crystal and Semi-crystal may be included
in the variety name of an
opal. These terms do only indicate the transparency of
an opal. They do not refer to its
structure since opal is near-amorphous and not
crystalline. In crystalline materials the atoms are arranged in a regular,
repeating, three-dimensional pattern.
More or less TRANSPARENT opals.
Under spot light you should be able to read a text (black on white)
through this kind of opal while it is lying directly on the text. This
becomes increasingly difficult the less transparent the opal is but also
the darker the body tone turns (e.g. Black Crystal Opal).
Transparent opals which are almost colourless are sometimes named Water Opals.
This term usually applies to
TRANSLUCENT Light Opals only.
These stones have a distinct cloudy appearance so that you are no longer
able to read a text (black on white) through this kind of opal. However
it should still be possible to discern the black text as an unreadable
Note: OPAQUE opals don't have a special,
additional designation in their variety names.