opal is known as a gemstone
showing many colours in a variety of
patterns. As a gemstone, opal has been known
for more than two thousand years, but has
only become widely available in the hundred
years since the discovery of deposits in
Australia. However there are many other
varieties of opal, all of mineralogical
interest, some of commercial and industrial
value. The latter types are usually termed
The relationships between these materials
can best be appreciated if we understand
the nature of opal or opaline silica itself.
The last term gives us the clue to its nature.
Opal is composed of silica (silicon dioxide),
although not all silica is opaline. Silica
occurs in many different forms in nature,
the best known being quartz, which itself
occurs in different forms, ranging from
rock crystal and gemstones such as amethyst
and citrine, to massive rock formations
such as quartzite
and sandstone, to sand on beaches and
crystals are well known, have their
own intrinsic beauty and may be very
quartz is used as a gemstone. Some people
believe that quartz crystals have certain
healing and magical properties.
Another common form of
quartz is known by the general term of chalcedony.
This is a microcrystalline form; that is
it is comprised of a multitude of minute
quartz crystallites visible only under a
microscope. chalcedony is a hard and tough
material which has a variety of uses. It
occurs in large quantities in chalk beds
nodules which, when broken, are seen
to be composed of grey to black material
with a thin white coating on the outside.
They are particularly well known in the
south and east of England where they have
been used from time immemorial.
Flint has an excellent conchoidal fracture,
so that very sharp edges can be produced
by the skilled craftsman. The early Celtic
inhabitants of England have left large quarries
in East Anglia, where they produced and
shaped the flint by skilled chipping and
flaking to form stone tools such as axes,
heads and so on. Being very strong and
resistant to weathering, flint has been
widely used as a building material where
it was abundant. Beautiful flint
churches of earlier centuries are still
a common sight in the south and east of
form of chalcedony which resembles flint
to some extent is chert. This often occurs
as layers in bedded limestones. Chert has
long been used in the ceramic industry.
The material is crushed to a fine powder
and added to the clay and feldspar when
making stoneware and porcelain articles.
Patterned or brightly coloured forms of
chalcedony are Less common, and have been
used as gem and ornamental materials for
thousands of years. Familiar stones in this
category are chrysoprase,
agate and onyx.
Silica also occurs in other less common
forms in nature. Many of these are only
formed at high temperatures and/or pressures.
The better known forms in this category
are cristobalite and tridymite, which while
not common in natural materials, are very
important industrially; they are particularly
important in the ceramics industry, especially
in relation to refractories.
Occasionally a natural form called melanophlogite
forms tiny cubes in volcanic areas. Other
rare forms, such as coesite and stishovite,
were created by enormous pressures generated
in rocks such as those around Meteor Crater
in Arizona by the impact of the meteorite.
Another form called keatite has been made
in the laboratory. In all, more than 20
forms of silica are known.
forms compounds with other elements in the
earth's crust and mantle. These are the
silicate minerals, more than 800 of which
are known. Most of these materials are uncommon
or rare; those making up a major part of
rocks in the crust number little more than
a dozen. Some of the commonest are feldspar,
mica, and olivine. Together with quartz
they make up the greater part of our
planet outside of the iron-nickel core.
The element silicon, at about 28%, is the
second most abundant material by volume
(after oxygen) in the crust of the earth
- that part 20-40 kilometres down from the
surface on which we live.
Opal differs from most other forms of silica
in that it contains water, the content of
which varies from about three to 20%, depending
in part on the type of opaline silica. It
may be noted that there are other hydrous
silica minerals such as silhydrite, but
these are rare. What then are these various
types of opal?
Our initial question (what is opal?) yielded
the answer precious or gem opal, which indeed
is the best known opaline material. Although
a scientific classification has been proposed
for these materials (Jones and the author,
and is given here,
we may confine ourselves at this stage to
describing the more commonly recognised