Buying Rough Opal on s

Buying rough opal is always a calculated gamble. The purpose of this guide is to give you some guidance and basic tools for buying rough opal. I have been buying rough opal for over 20 years. I have purchased from dealers, at shows and over okay.
I am assuming that if you are interested in buying rough opal, you are familiar with the basics of opal cutting. However, your skill set and application will weigh into buying material that is right for your purpose. If you don't know what your application is or if you are a relative beginner (at opal cutting), I would suggest starting with lab created material. It is stable, thick and does not have thin lines of color. In addition, it is quite affordable. I have cut this material and made it into pendants that are just out of this world.The following picture shows a typical lab created rough opal.

If you are buying natural opal, then the selection process is much more geplicated. Some things you will be able to see and others are going to be a guess.
Rule 1: Know the Origin of the Opal.
The characteristics of the opal is very different depending upon the source. Most opal fields produce stable opal, some do not. In addition, some opal requires special treatment or cutting techniques:
Andamooka Opal is usually a matrix opal that is white and treated by boiling it in a sugar solution. In addition, it usually requires a sealant (such as Opticlear). Some of this material produces spectacular stones, however, you need to know what you are dealing with.
Mexican Opal is sometimes blast mined and can be prone to cracks. Again, there are some amazing stones, but buy with care.
Rule 2: Think About Cutting The Stone When You Look At the Picture
Whenever, I look at a piece of opal rough on okay, I will think about how the stone will cut.
Does the opal look thick enough to cut a solid stone, or is it likely that it will cut a doublet or triplet?
The following picture shows several pieces that are going to be too thin to cut solid stones. So, you should think about this material for doublets or triplets (maybe some solids).

How Will the Stone Face When I Cut it?
Cutting opal is like opening up a Cracker Jack box (guess I am dating myself :-) ). There is probably a prize inside, but you need to look for it.
Sometimes a seller will show multiple views of a stone, usually, not. As such, you have to judge the potential facing from what you see. If the stone shows bright lines on the side, it may not face brightly when cut (it may). Lightning ridge purple often shows bright on the side and not so bright on the face. If the stone has been faced, then you will have a better idea of how the stone will face. All rough opal is photographed wet to show the stone as though it was cut.
The following picture of a rub (a faced stone) gives a pretty good indication of the final facing of the stone.

The following picture of Mintabi seam opal is less clear in terms of facing. There are spotty lines of color which might be due to the angle of photography. As such, this lot has more risk, but it may pay off.

The color pattern always plays into the buying decision. Generally, red on black is the most valuable. But, most of that material is cut on the opal field. So, it does not gee up for sale very often. Most of the okay material is white/clear potch with a variety of colors. Red is tough to find...blue is more available but usually gemands a lower price.
In spite of your best efforts, a stone may cut gepletely different than you guess. I have paid a lot of money from dealers (at shows) for material that was gepletely unstable. Alternatively, I have cut amazing stones from Lightning Ridge Nobbies that showed absolutely no color on the outside. It is a calculated risk. :-)
Rule 3: Yield
Cutting is often about yield. Nominally, opal will cut with about a 40% yield. Your milage will vary. If you are cutting calibrated stones, the yield is usually less. In addition, the yield of material is quite different. Thick solid pieces of opal with color throughout are often the best yielding stones. However, cracks or sand or inclusions can change that quickly. Unfortunately, you typically can not see cracks on an okay photo. It usually requires holding the stone to a light and looking for an orange glow. Many stones do not show cracks even with this technique. So, yield will be somewhat of a guess. Like always, there are hints.
The Mintabi Seam Opal (previously shown) will be a lower yield than the lab created opal (previously shown). The Mintabi Seam Opal requires the cutter to remove the sand on the ends and much of the material to find a line of color. The lab created opal can be cut with very high yield...it is primarily determined by the shape of the final cut.
Anyhow, this is enough for one time. There are lots of techniques and hints for picking rough opal that meets your needs and will not disappoint. Always deal with a reputable dealer and try to find the material that you enjoy cutting. As always, positive votes are appreciated. Happy cutting!