AussiePOD: Aussie Picture Of the Day

Opal is a non-crystalline form of the mineral silica that, despite its amorphous structure, displays an amazing degree of internal organization. Opal is related to its more commonly found but highly crystalline cousins quartz and agate, and is formed from amorphous "balls" or "lumps" of silica rather that from ordered, naturally-faceted crystals.

One thing you should know about opals is that there are partially man-made stones known as doublets and triplets. These stones are slices of opal glued to a black backing, designed to imitate the appearance of a solid black opal. Adhering the slice of opal to a black backing causes the colour to become much darker and more vibrant. However doublets and triplets are generally much cheaper than solid black opals as they only contain a small amount of genuine opal.

You can usually identify doublets and triplets by looking at the side of the opal - if the stone has been adhered together you will notice that the line where the coloured opal and the black backing meet is perfectly straight.