OPALFIRE.INFO

Guide to Rough Opal Parcels

Opal Parcel Information

Opal Parcel Information

A rough opal stone, or opal rough, usually consists of a combination of several
materials. These are matrix, potch and precious opal.

Matrix is the non-opal stone in which the opal was deposited, and from which it was
mined. This is the "host stone" for the opal. This may be any one of various materials,
depending on the source of the opal. This material may be sandstone, ironstone, rhyolite,
andesite, quartz, basalt or any number of other minerals. Usually most or all of this is
trimmed away in the opal cutting process, so the ratio of opal to matrix is an important
factor in purchasing rough.

There is a variety called matrix opal, in which the opal is in very small or fine
particles mixed throughout the host stone, so the entire piece is polished and the opal
sparkles are sprinkled across the surface of the stone. The important factor there is the
concentration, brightness and colors of the opal particles. Another factor is the beauty
of the matrix host stone. Often the matrix is treated to
darken it for a brighter relative display of the opal particles. This sort is often
treated with a sealer material to produce a smoother surface and better polish.
This variety is found in Andamooka Australia and from Honduras.

Some opal material is cut leaving just some of the matrix showing in the finished stone.
The opal may be in large nodules, surrounded by the matrix. This is often cut in a window
style, with a bit of the matrix left around the precious opal. In Mexican opal, this is known as canterra.

Potch, or common opal, is solid opal of any color which does not show the sparkly play or
apparent movement of color as the stone is moved. Opal potch may come in almost any
color, and some is quite attractive if polished into a cabachon. If transparent, it may
be faceted as well.

There is a variety of common opal which is considered separately. This is known as fire
opal, a rather loosely used term which should truly only refer to clear or translucent
material in the yellow to orange to red color range. The term fire refers to the color of
the body of the opal itself, thought of as the color of flames. In today's market, any opal with
a play of color is referred to as fiery or fire opal or as the fire in the opal. This is
a common term for the play of colors seen in opal, so confusion abounds. Nonetheless,
true fire opal is a variety of common opal described by color and transparency.

Precious opal is opal which shows a play of color in some pattern which changes as the
stone is moved. Naturally, the brighter the color and the more colors present, the
better. Varieties of pattern and relative worth of colors and patterns will be discussed
below, along with a discussion of the various body colors, the color of the stone
separate from the color and brightness of the play of color.

Opal rough is sold in parcels, the Australian term for any amount of opal being offered.
A parcel may range from a single stone to a batch of hundreds of stones. Quality may
range from "rabbit rocks", potch or matrix (or both) which is useful only for slingshots
and target practice, to brilliant crystal fiery sparklers which will knock your eyes out.

Usually there is a range of quality or grades of opal in a particular parcel, although
some dealers grade parcels pretty strictly, based on what they can see of the color and
yield.

Total production from a mine is known as mine-run. Often offered, but seldom true. Almost
all opal parcels are the result of grading and sorting the material from a mine. The top
10 percent or so of mine production is seldom seen except by purchasers of large
quantities of opal rough, by buyers who deal directly with miners or miners' agents. This
top quality material is usually purchased by European and Japanese buyers, who pay much
more than the market will bear in the USA.

A piece of precious opal rough is valued by the percentage of precious opal present in
the stone. If the stone is almost completely precious opal, it is known as skin-to-skin,
referring to the outer layers of the stone, which is often a thin layer of potch or a sandy crust, with
the precious opal hiding below. Often a miner will pinch or nip off a bit of the skin in
a few places to see the color.

A piece of opal may also contain bars or bands or layers or sections of precious opal
surrounded by potch. This is often referred to as P & C or potch and color. The size of
the precious part relative to the potch or the matrix present is the important thing
here. Banded or layered sections of precious opal may be thick or paper-thin. You may be
able to cut a stone which is solid precious opal if the sections or bands are thick
enough, and if the color band is relatively flat or straight. Sometimes a band of color
will wander through the stone, curving or bending, and the band may thin or play out
within the stone. Thinner bands may be cut with only the precious color showing on top,
while the bulk of the stone may be potch.

The relative value of a piece or parcel lies in the percentage of weight of finished
stone it will produce. This is known as yield or return. Most opal rough will return a
relatively small percentage of the original stone. Most of the stone will be ground away
in the shaping, or pre-forming process. The amount depends on the original shape of the
stone as well.

Some people prefer to cut calibrated stones, which are a standard size, shape ond
thickness, and will fit into pre-made ring and pendant findings, so the jewelry mount
does not have to be made specifically for the stone. More opal, often including precious play
of color, will have to be ground away. This returns a smaller yield than if the stone is
a freeform shape.

Freeform means that the stone has been cut, shaped and contoured to the boundaries of the
precious opal section rather than a specific size and shape. This will naturally return a
larger stone which will not be perfectly round, oval or any other regular geometric
shape.

Yield may be as small as 5 to 10 percent or rarely, may be 50 to 70 percent. Higher yields may
be obtained by carving the stone, but seldom over 70%. A bottom line average is about 20
to 30% although many dealers will claim higher percentages of return for a particular
parcel. Keep grains of salt handy.

So a parcel of P & C should sell for less than a parcel of skin-to-skin opal of the same
size and color quality, since the yield will be less. Of course, often it is difficult to
determine from pictures, or even by personal inspection, just what the stone will return,
since only Superman can see into the stone to determine what happens to the color.

Various opal dealers have claimed to have "X-ray" eyes and suggest that you trust them.
This is not recommended, even if the dealer offers a return guarantee. All that guarantee
buys you is a chance to see the stones in hand rather than just a picture.

Always decide if you want to keep the parcel before cutting or nipping or processing the
stones in any way. No dealer will accept returned parcels if they have been altered in
any way, such as nipping or grinding to expose the color. If you cut it, it's yours.

Often opal is nipped by the miner or dealer to show the color inside and sometimes rough
will be faced or edged. This means that a section of the skin has been ground away to
expose the fire below, either ground across a large section of a flat surface, or the
edges may be ground away to show the number and thickness of the color bars or bands.
This is called facing the opal.

If a face is ground to expose a broad section of color, the stone is referred to as a
rub. Usually most of the matrix has been ground away as well. This is a good way to be
more certain about what you are getting and is often recommended for beginners. Actually
it is a more useful strategy for experienced cutters, as a rub may have a very thin layer
of color on its face and a less experienced cutter may grind through it in the sanding
and polishing stages.

Rubs will generally be more expensive than a parcel of unrubbed opal, as there are fewer
surprises. This is good and bad, as part of the joy of cutting for some people is the
mystery of what's inside. Others are not so adventurous.

Another way that rough opal is offered is as pre-forms. This means that the individual
stone has been ground into a close approximation of its final shape and size, exposing
the fire and removing most of the matrix and potch. All the cutter has to do is to finish
the final shaping and surface smoothing and then polish the stone. Naturally this returns
the highest yield for weight purchased and is priced accordingly higher.

Preforms are a good way for a novice cutter with a light touch to produce a good looking
finished stone, but a heavy hand will leave a beginner with an expensive pile of dust and
a much smaller stone than the preform was expected to produce.

Other factors in valuing a parcel include size of the individual stones, number and
brightness of colors, stone shapes, the patterns the colors form and the body color of
the stone.

Other factors in valuing a parcel include size of the individual stones, number and
brightness of colors, stone shapes, the patterns the colors form and the body color of
the stone.

Body color is the color of the stone itself, separate from the play of color. Opal forms
in almost every color but most range along the grey scale from black to white. Other hues
are generally not as common or as valuable, except in their rarity. Browns and yellows
and blues are generally not so highly valued as the darker grey scale opals, but some may be as
valuable or more so that the mid to lighter greys and whites. Precious blue opal from the
Southwest USA is quite rare and commands a premium if strong play of color is present.
Grey-blues from Australia are much more common and not so valuable. Dark greys and true
blacks are at the top of this scale, along with crystals.

Crystal and jelly opal are another story in themselves. Crystal just means transparent,
so that you can see through it. Jelly means that it is a little cloudy or translucent.
Crystal comes in various shades and hues, as does jelly. Crystal is naturally more
valuable since the play of colors can shine more brightly from the clear base, but value
is very dependent on the strength and brightness of the play of color.

Patterns in the play of color are another major factor, deserving of their own guide. I
will be producing a guide to colors, patterns and the terminology used to describe them,
complete with pictures and definitions. Soon come, as they say in Jamaica. For now,
suffice it to say that the terminology, as used on eBay and other web-sites, is very
loose and is often misleading. For example, Harlequin and flagstone are similar but very
precisely different patterns. Harlequin is very specifically limited to perfectly square,
evenly sized sections of various colors laid out in a checkerboard pattern. Old Aussie
miners called it Tartan, but that term has been mostly abandoned. Harlequin is one of the
most abused and mis-used terms in opal advertisements today. Watch for more information
on patterns, coming soon.

Brightness is another term misused often. Most dealers describe their opals as no less
than 4 to 5 on a scale of 5, but few people have actually seen, much less sold, opal near
to 5 in brightness. And you certainly won't see it priced at under a thousand dollars per
carat, even in rough form. This quality of opal is seldom seen in the USA.

Stability is another important factor, although it is more difficult to determine from
rough. Naturally if you can see cracks or crazing, the opal won't be for sale, or
shouldn't be used for anything but carving or specimens - maybe not even then. Some
source countries have better reputations than others and locations within a country or
even within a region may vary greatly in stability. But a lot of what is called fact is
actually conjecture or myth or rumor. There are real geological reasons for opal
structure, and some layers of a continuous formation may be much more fragile than
others.

Knowing your dealer is extra important here, as is a return guarantee. Usually opal is
photographed wet for internet display ads, and lots of gem show dealers display opal in
water to show the colors better. Water is roughly the same refractive index as opal, so
the surface roughness of the opal disappears when wet and the opal displays colors much
closer to the polished finished product. But water also disguises crazing and cracks, so
always let your opal dry and inspect it closely before committing to a purchase. Most
dealers will allow a return of purchase price or a credit towards another parcel if you
decide you don't want a particular batch. Some will not. Be sure to ask before purchasing.

Opal primarily comes from various regions of Australia. Many other countries produce
precious and common opal as well. Precious sources of note include Brazil, Mexico,
Honduras, Indonesia, Ethiopia and various regions of the USA. Common opal comes from
dozens of countries and areas and is available in almost all colors.



Here is a section which describes a relatively new opal source, Honduras. This was
written by my friend David Sahadi, dsahadi on eBay, and he graciously allowed me to
include it. Thank you, David.



I don't a whole lot about the nodules. The people who mine it don't really know a lot
about it. People have been aware of it for a few years but it is really just now being
commercially mined. There is a company that managed to secure the mining rights for all
opal exported out of Honduras and they are the ones mining it. I bought about 2 kilos of
the first lot to come out.

All honduran opal is volcanic. As opposed to Australian opal that created as a result of
a silca rich water table rising and falling depositing opal in spaces and cracks in the
geology over millions of years, honduran opal came about as a result of silica rich steam
venting, a relatively quick process I would think is still on going at some level. The
Andesite matrix is one of the host stones.....you can see the opal deposits in the
matrix...solid honduran opals are generally cut from large spaces where the opal
collected. Personally I prefer when you find the opal mixed with the matrix. It is a bit
less expensive and there are no stability problems. The honduran solid opal will show
stress fractures....and when you cut the opal you need to sometimes split the stone where
you find a fracture. Once cut an polished the stone seem to be stable. There are two
other types of relatively well known opal formations from Honduras...one is black
seam....as the name suggests this a seam of opal that is sandwiched by black basalt. The
other probably best known is the Black matrix. This black matrix is black basalt matrix
that appears to be shot through with very colorful opal. I have some large pieces of this
material and it is beautiful. Obviously this material has no stability issues. All of
this material is available cut on a commercial basis as cut stones and in jewelry.

That brings us to the nodules. This material is found in clumps and the nodule fingers
are broken away from this clump. The nodules can range in size from a pea to about the
size of a grapefruit. It appear to me to have been gravel exposed to a super heated steam
vent. The reason I say this because of the gravel looking nature of the matrix. The
chambers in the material appear to have been blown and the gravel its self looks to have
been glued together with the opal. I have discussed this with a partner in the operation
and this jives with the observation made by their geologist. The material as I mentioned
is new to the market and so cutter like yourself are the ones coming up with creative
ways to use and cut this material. As I suggested, I found that cutting the opal with the
matrix offers some very interesting finished stones. I would think of cutting this
material in much the same way Mexican canterra opal is cut.

MK-my thanks to David for this explanation...check out his auctions, dsahadi on eBay...

This is a brief outline of some of the factors surrounding opal parcel purchases. There
are so many other factors, some clear, some rather murky, which are involved. Experience
intuitively teaches us much over time, and some of it is hard to put into words. You
learn more with time and there is no substitution for experience. If you are a beginner,
purchase small amounts of cheaper parcels and proceed slowly. There are lots of guides
and manuals to teach cutting techniques, but you can't learn to cut from a book, and no
book can teach you to judge rough opal. You can get lots of advice, here and elsewhere,
but nothing can teach you the feel of opal in your hand as you put it to the grinding
wheel. You just gotta do it.

Happy cutting,

Mike Kelley

arthursonn on eBay