Opal

Opal is the birthstone for October and the national gemstone of Australia, renowned for its vibrant play of colors, referred to as opalescence or play of color. Opals are categorized based on their base color into white opal, black opal, fire opal, and crystal opal, with black opal being the most valuable. Boulder opal, which is opal cut with its host rock, is less expensive but can still exhibit a strong play of color. The play of color in opals comes in three patterns: pinpoint (small dots), broad flash (large patches), and harlequin (angular shapes), with red flashes being the rarest and most valuable. The quality of the play of color is the primary factor determining the value of an opal.