The October gemstone is OPAL, which is a 4-letter word. OPAL is a mineraloid gemstone composed of hydrated amorphous silica, which is a type of silicon dioxide. It is usually found in sedimentary rocks and is known for its unique play of color, which is caused by the diffraction of light from the tiny silica spheres that make up the stone. OPAL is the birthstone for October and is believed to bring luck and good fortune to those born in this month.