THE MOST POPULAR AND PRICED ARE THE AUSTRALIAN BLACK OPALS
Also the white opals are very popular and vendible because they are more affordable compared with the black ones; White opals can be milky white and semitransparent, which is why they are normally displayed on dark or black surfaces. Other very popular currently are the crystalline that are transparent but beautiful and they definitely have to be displayed on black surfaces to appreciate their colors.
WHAT IS POTCH? POTCH IS WHAT PRECIOUS OPAL IS FORMED ON. IT CAN BE GREY, BLACK OR EVEN WHITE IN COLOR.
The majority of common opal mined is called Potch, so it is basically common opal with no color. Potch is a hydrated amorphous form of silica and is known as a mineraloid. It is not classed as a mineral. The color is formed in the internal structures of precious Opal. Precious ones will diffract light creating a spectrum of colors whereas potch has no diffraction and no color.
BOULDER OPALS ARE A UNIQUE STONES UNLIKE ANY OTHER
Opal is a material that forms within the cracks and crevices of different types of rocks. Often, the veins of opal within a rock are thin and winding. Quite often the formation is not quite big enough for a stone of its own, though it is still dazzlingly beautiful within the rock. At some point, someone had the good sense to realize that the rock and opal together can make a beautiful stone, which is precisely what boulder opals are.