Definition of "opal" []

• A mineral of hydrated silica. (noun)
• A gemstone made of this mineral, noted for its rich iridescence. (noun)

American Heritage(R) Dictionary of the English Language, Fifth Edition. Copyright (c) 2011 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.

• "Although opal is not a conclusive indicator of a wetter Mars, it does suggest some interesting scenarios."
• "The most durable opal is a solid piece, free from fractures."
• "The very word opal comes from the Sanskrit upala, meaning “precious,” which I guess says it all!"