Opal Calibration Chart

When you first get to know about cutting opals, the challenge is to be able to cut the stones in a perfect oval.  After a while your eye gets used to it.  You start using your fingers as a human lathe and you know just the right ‘touch’ as the dop stick and opal touches the grinding wheel. Of course, after you have finished the front of the stone, you turn it around and re glue it to the dop stick and you can correct the oval from the back to make sure its perfect.  However, before you gain this skill, its a good idea to get one of this calibration chart, for two reasons.

opals ready to be fashioned with dopstics

opals ready to be fashioned with dopstics


Opal Calibration Chart-for cutting opals into specific sizes

Opal Calibration Chart

The value of a calibration chart in cutting opal

Not only does this chart teach you how to cut perfect ovals but you will notice that it is separated into many different shapes and sizes. The reason for calibrating opal, or for that matter, any other cabachon (round topped) gemstone is that you can later fit it to standard shaped jewelry.

The reason that this is so valuable is that it could very well take away a lot of the manufacturing costs if you want to take your opal cutting to the next level and fit it to rings, pendants, earrings, bracelets, cuff links, tie bars, tie tacks and any other jewelry item you can think of, you will do it much more cheaply if you make sure your opals are specific sizes.  This is done with the opal calibration Chart.

What are Calibration Charts Made of?

Well, this one is made of plastic and my opinion is that the plastic ones are the best because they are more flexible and easy to handle. You will find of course that there are quite a lot of the sizes in the calibration chart that you really don’t use much.  The main sizes that fit standard jewelry fittings are as follows:

6×4,7×5,8×6,9×7,10×8,12×10,14×10,18×13 and if you are into belt buckles and bolas you might need to go to 25×18 and 40×22.  In regard to opal, its usually boulder opals and large opal mosaics that would be used for these larger opal sizes.

opal calibration chart - very high domed crystal opal ring with diamonds

very high domed crystal opal ring with diamonds


This Stone in the ring to the right, Does not Need a Calibration Chart because the Ring has been Hand made. Calibration charts are only needed when you want to do multiple settings of low priced jewelry. The Solid high domed crystal is a high grade gemstone surrounded by diamonds, set in 18k yellow gold. The value of the stone warrents the time and effort it takes to produce a ring like this.  But not everyone can afford to spend $10,000 or so (This one is $15,000) so calibration charts come in handy for this purpose.

How do you Use the Opal Calibration Chart

Keep the calibration chart close to your grindstone. You could also keep a set of calipers close by (this is a brass tool that you can open and close to measure the mm’s or inches of your opal. Brass is used because the tool often gets wet and it will not rust.  As you can see, the measurements on most calibration charts are in mm’s so its a good idea to have a chart comparing the mm’s with inches, if you use inches in your country. (As is the case in the USA)

The procedure is quite simple. You just have to keep checking the size and shape of the opal against the calibration chart to make sure its a perfect oval and is the right size for future jewelry castings. If you would like more information on where to get castings for your stones, please leave a message on this blog

boulder opal roughdop stickOpal Calibration Chartopal cuttingopal polishingopal sandingrough opalwhite opal rough