Scorpio's birthstone is OPAL, a four letter word. OPAL is a mineraloid gemstone composed of hydrated amorphous silica, which is found in sedimentary rocks. It is known for its unique play of color, which can range from white to black, and can display flashes of red, orange, yellow, green, blue, magenta, pink, and other hues. OPAL is the traditional birthstone for October, and is associated with Scorpio, the zodiac sign for those born between October 23 and November 21.