The wonderful thing about boulder opal is it's strength and vivid colors.  The white opal from yesteryears that was about 1mm thick and pure crystal was not resistant to wear.  Boulder opal on the other hand is similar to turquoise.  On the Moh's scale it is 5.5- 6 in hardness due to the ironstone host rock.  And, as far as I am concerned the ironstone looks like chocolate!