October - the birthstones for October are the opal and the tourmaline.
Opals and tourmalines
There are two distinct types of opal, common and precious. The way the silica particles form determines which type. In precious opal, silica particles are packed in regular rows and layers. Moving the stone causes light to diffract, or split, as it grazes the opal surface. This light diffusion shows iridescent flashes of green, blue, aqua and sometimes yellowish or red colors which are referred to as "fire".
The gemstone Tourmaline is the official birthstone for October as adopted by the American National Association of Jewelers in 1912. It also the tradidional birthstone for October, the stone for the Zodiac sign of Leo, and the accepted gem for the 8th wedding anniversary.
The name Tourmaline comes from the Sinhalese (Sri Lanka) word tura mali which translates as the stone of mixed colors. These stones are 7 to 7.5 on the Mohs’ scale of hardness.
Tourmaline is available in a wide variety of colors from black to bluish-black, dark brown, yellow, medium brown, blue to neon blue, lime to dark forrest green, red and reddish purple, yellow, pink, and colorless.
Bi-colored and multicolored tourmalines may be green at one end and pink at the other, watermelon tourmalines are green on the outside and pink on the inside. Some stones are dichroic meaning they appear to change color when viewed from different angles.
The most expensive tourmalines are the blue indicolite, green verdelite and pink rubellite. (Information from www.bernardine.com.)