These are a pair of natural Coober Pedy Crystal Opal, mounted on an Ironstone backing making them Doublets. They come from the mines at Coober Pedy in South Australia. As they are natural stones you will see that there is a small difference in them. Normally it is very hard to find Opal that matches, hence they fetch a slightly higher price.
If you see an opal on this website that you might like to have as a pendant, please contact Peter to discuss if it is possible with that particular stone.
You also need to state if you wish to have a bail fitted or not. If you do, then you would need to be charged for the cost of metal, silver or gold.