Crystal Boulder Opal is often lighter in color, and usually either purple, blue, green, pink and sometimes rainbow.
The sandstone is more fragile than ironstone boulder opals.
The precious opal found is, like the name says, crystalised; apposed to ironstone boulder opals which are not.
Crystal opal is transparent, meaning that light can pass thru it.
It is therefor recommended to set them with some sort of backing or bezel.
This traps the light in to ensure your opal is always showing off at its best colors, pattern and brightness.