Boulder Opal: Ten Things to Keep in Mind when Buying

Boulder Opal: Ten Things to Keep in Mind when Buying

Ten Things to Keep in Mind when Buying this most precious gem from the Queensland Australia opal fields

A. Boulder Opal

Boulder Opal: Ten Things to Keep in Mind when Buying
Kernal of red in boulder opal
Boulder Opal: Ten Things to Keep in Mind when Buying
undulating boulder opal

  1. What IS Boulder opal? This is the first of the ten things to keep in mind when buying boulder opal. First, boulder is a variety of the black gem.
  2. The actual terms were defined in the current opal nomenclature and a correction was made from a previous understanding that the term black opal only applied to Blacks from Lightning Ridge, which is the traditional source of supply.
  3. However, it has become known that Boulder opals can also be described as black opals because of their dark background.
  4. But I thought that Boulder Opal had a brown background, not black! Yes, it’s true that boulder opal comes from a ‘mother’ or ‘matrix’ of boulder ironstone. The stone is cut in such a way that the ironstone is left in the background to give the thin veins of color a base. However, the brown ironstone is so dark at times that it creates the same effect as black opal potch does in opals from Lightning Ridge. Hence it can be described as a variety of black
  5. What is the color of Boulder opal? The answer is ANY color. Blacks can be blue, green, yellow, red, orange, purple and of course, a combination of all these colors. Remember, a good quality black opal is not black. It has the ability if you could call it that, to suck all the colors from the rainbow and display them in all their variations.
  6. <How do you know a good quality Boulder? The main thing is to allow your eye to perceive what is brighter and more dramatic. The blacker the background, and the brighter and more stunning the foreground is the best indicator of a good black boulder opal.
  7. What is the difference between a black Boulder and a dark Boulder? Well, they are really from the same family. All black opals are dark opals but one with a background that is grayer is called dark, not black. For the purpose of identification, often dark opals will be listed as blacks because not many people know the technicality, but the fact is that a good quality black gem is always very dark in the background.
  8. How come some Boulder opals with creamy or lighter colored sandstone backs are still called blacks? This is a very good question, and the answer is that sometimes there is a strip of really black potch or dark ironstone directly under the foreground color, giving the stone a really dark background. But then under that strip of dark material, there is a strip of lighter colored sandstone. This doesn’t matter because the important part is that the background of the color is still black if the color is sitting directly on the dark area.
  9. Why do boulder opals from Australia have such a good reputation? Because Australia, being a very old dry continent, has opals that have dried out over a very long period of time and are less prone to cracking or crazing. There are some very beautiful stones from Africa and the USA and South America, to name a few of the countries but generally they are only appropriate for specimen purposes, not to be set in jewellery. This is only a general comment because there are opals from these countries that have not cracked but you have to be very careful because they may in time. In the case of Boulder opal, the ironstone gives the stone very good support and so it is very unusual for a boulder gem to crack or craze.
  10. Are all Australian Boulder opals free from the problems of cracking or crazing? Generally yes. If a boulder opal is going to crack or break, it always happens in the cutting process. Having said that, only deal with reputable opal merchants with many years of experience because unscrupulous dealers will try to sell you an insecure stone for a top price. Opal cracking is not a big problem but it does exist.
  11. Is there any particular shape that is preferable in Boulder opal? The traditional shape for black opal is oval with a high dome, and this has resulted in a lot of good quality opal being wasted because cutting an oval out of a piece of rough opal is not very economical. In recent years the public has woken up to the fact that free-form black opals are just as, if not more interesting to look at as the ovals. Black boulder opals have never succumbed to the tradition of only cutting ovals. Most boulder opals are cut as free forms.
  12. What is a Boulder Opal undulate? Well, you won’t hear that expression used anywhere else but from this writer, as it was more or less invented by myself to cover an area of opal finishing that has not been addressed, as far as I know. In recent years to save wastage of a gemstone that is fast becoming extinct, the practice has been to use dental drills to clean up the surface of the black stones by removing any imperfections, thus creating a gem with ‘hills’ and ‘valleys’ or ‘undulations. These pieces are really magnificent and I have applied this term to describe them.

A final word of caution:

Avoid the trap of getting something that you thought was a solid boulder opal when in actual fact was an opal doublet that looked like a real boulder opal. If you have any questions about this subject or need some help in identifying a stone you have in mind to buy, please take advantage of this blog. Maybe you can add something else to the knowledge base.

some nice pics of boulder opal and others

B. Crystal opal (coming soon)

C. Black Opal (coming soon)

D. Rough opal

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7 thoughts on “Boulder Opal: Ten Things to Keep in Mind when Buying

  1. Sorry for the caps – some bug!

    I have a 90 x 90 x 40mm boulder opal weighing about 500 grams. I am in the southwest of england. can you suggest how I may go about getting it appraised and sold? kind regards, Edmund

    1. Send me a pic of the stone to

  2. Thanks Peter. I will make up 3 small representative samples for photos as you described. Is the actual size of the rocks relevant? Some are more like chips and some are large pebbles. Yes I can get to Hatton Garden easily, do you know an opal trader there? – thought they would only be interested in very high quality stones? Deborah

    1. Deborah i know a nice lady who used to have an office in Hatton Garden. her name is Kate McLaughlin. She used to come to australia years ago. not sure if she is still in business there or not but i will email her and let you know. in the meantime, send me an email to peter at opalmine dot com and if she answers i will forward her details to you. Kate knows opal very well but as you suggest she usually only deals in top quality gems. however i am sure she will give you an opinion if you mention my name. peter brusaschi. you could also try to give this lapidary club a call:

  3. Hello, my husband was an amateur who cut and polished opals as a hobby. He died last year. I have inherited several kilos of opal rough, most of it Coober Pedy white opal and some lightning Ridge black opal. Very rough indeed, it is hard to see colour in many of the stones. I live in the UK near London and would like to sell this rough to members of a local lapidary club but I have no idea what it is worth. All I can find on the internet are individual good quality pieces of rough stone with obvious colour but these are mixed bags which would take lots of work to grind and polish. Can you help please with an estimated value per 100 grams? thanks very much, Deborah

    1. Deborah the first thing you have to do is grade the opal. just put aside all the pieces that have color in them. separate the ones with a lot of color from those with less color and others with a lot less color. then you will need to take a picture of each of the parcels and reduce them in size to around 200 kb and post them on this site. please dont paste huge images. Where do you live in the UK? Are you anywhere near Hatton Garden in London? peter

    2. Deborah,
      Hi there, don’t know if you still have any of this opal left, but there are a few lapidary groups around london, essex, ( cambridge etc, and lots of individuals who will be willing to buy as well.
      If you search lapidary on Facebook there are lots of clubs.
      I’m in Cambridge and would be interested in whatever you have left as well, I am part of an online lapidary group, and I am just starting out so hence i was looking at this site.

      My email if its of any use is

      I wish you all the best


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