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| BOULDER
OPAL, Untreated |
Cut
with the mother rock intact, each stone's
shape conforms to the shape of the rough.
Therefore, each boulder opal's charm lies
in its unique shape and brilliance. |
The dictionary defines the word boulder
as "a large, rounded stone."This
type of opal is called "boulder
opal" because it is found in
the ironstone boulders mined in the
state of Queensland, Australia. Boulder
opal forms as layers within the mother
rock (matrix) and is polished with
this matrix attached. There are "full-face"
pieces that have opal on the entire
surface, and "partial-face"
opals that show patches of mother
rock when viewed face-up.
Opal has
been mined since the beginning of
the 20th century in and around the
three localities of Yowah, Quilpie,
and Winton in the state of Queensland.
At first, the opal portions were generally
removed from the matrix and polished.
Since the 1960s, however, the previously
discarded portions containing opal
have been polished with the matrix
intact, resulting in boulder opal.
Since the 1980s, pieces have been
polished to follow the shape of the
mother rock, creating gems of various
shape and thickness. In a way, the
transition to boulder opal symbolizes
the exhaustion of gemstone resources. |
When
a cutter polishes a boulder opal,
it is given a free-form shape to
preserve as much of the opal as possible
while using the mother rock effectively.
On rare occasions, stones are repolished
into standard shapes such as ovals.
The boulder opal as shown in the
photograph on the
next page is a
full-face opal with a flat surface
that exhibits distinct patches of
colors. This is called the "play-of-color" effect,
and it is the key factor in the quality
of an opal. Each piece's unique form
and brilliant colors create the charm
of boulder opal.
Some Mexican opals and light opals
with high water content may develop
cracks if they dry out, but boulder
opals rarely have such problems.
Opals are fragile, however, and care
must be taken not to strike them
against hard surfaces. Also, assembled
imitations such as opal doublets
and triplets are often sold as boulder
opal, making it important to choose
your opal from a reliable jeweler. |
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Brooch,
Gold
Boulder Opal 1pc
Estimated Weight:2.30 ct
Untreated
Diamond 9pc
US $8,000 |
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