Each month of the year is traditionally associated with a gemstone. For those born in October your birthstone is the shimming opal. This semi-precious gemstone has a rainbow-like iridescence which changes with the angle of observation. There are five main types of opal: White or Light Opal, Black Opal, Fire Opal, Boulder Opal, and Crystal or Water Opal. 
In Ancient Rome opals signified love and hope. It was once thought that opals were the product of lightning that fell from the sky during thunderstorms. In reality, they are the product of the torrential rains which drench the ground in Australia's desert outback.