from The American Heritage® Dictionary of the English Language, 4th Edition
- n. A mineral of hydrated silica.
- n. A gemstone made of this mineral, noted for its rich iridescence.
from Wiktionary, Creative Commons Attribution/Share-Alike License
- n. A mineral consisting, like quartz, of silica, but inferior to quartz in hardness and specific gravity, of the chemical formula SiO2·nH2O.
- n. , (genetics), (biochemistry) A colloquial name used in molecular biology referring to a particular stop codon sequence, "UGA."
from the GNU version of the Collaborative International Dictionary of English
- n. A mineral consisting, like quartz, of silica, but inferior to quartz in hardness and specific gravity.
from The Century Dictionary and Cyclopedia
- n. A mineral consisting of silica like quartz, but in a different condition, having a lower specific gravity and hardness and being without crystalline structure: it usually contains some water, mostly from 3 to 9 per cent. ; ; ; ; ; ; ;
from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.
- n. a translucent mineral consisting of hydrated silica of variable color; some varieties are used as gemstones
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!
"The opal is oblong," the crystal-gazer continued,
The story of the king's opal is the one I know, and though it sounds exactly as though it had been made out of a dream, it is none the less interesting for that, anyone will admit; and this is the story:
The town is known as the opal capital of the world because 90% of the world's finest opals are mined there.
The opal is a beautiful stone which seen at different angles and in different lights seems to glow with various colors.
Of all precious stones, the opal is the most lovely and commonplace.
Frost's Laws and By-Laws of American Society A condensed but thorough treatise on etiquette and its usages in America, containing plain and reliable directions for deportment in every situation in life.
Mine, naturally, was the least expensive; it was an opal ring -- the opal was my favourite stone, because it seems to blush and turn pale as if it had a soul.
Hydrated silica, commonly known as opal, has been found across large region of Mars.