The "What Is It?" page is our monthly challenge. Below you will find a photo or a description of something related to geology, gems, minerals, or fossils. If you can guess what it is, click the Contact Us link at the bottom of the page and send me an email telling me what you think it is. The first person to answer correctly gets their name posted on this page. Answers will be given at our monthly club meeting.
The "What Is It?" Challenge for November, 2011:
This month's challenge is not a gemstone, but some
specimens can be polished and used as gems.
What is it?
What are the crinkly edges called?
Wanna take a guess at it? Email Me and submit your answer.
The "What Is It?" Challenge from October, 2011:
Our winner for October is Paul Howell.
I'd like to point out that Paul is one of our Junior Members. Way to go, Paul!
when viewed under natural daylight to another color
when viewed under incandescent light.
What is it called?
Where was it originally mined?
For whom was it named?
The most sensational feature about this stone … is its surprising ability to change its color. Green or bluish-green in daylight, alexandrite turns a soft shade of red, purplish-red or raspberry red in incandescent light. This unique optical characteristic makes it one of the most valuable gemstones of all, especially in fine qualities.
Alexandrite is very scarce: this is due to its chemical composition. It is basically a chrysoberyl. It differs from other chrysoberyls in that it not only contains iron and titanium, but also chromium as a major impurity. It is this very element which accounts for the spectacular color change. Only chrysoberyls displaying a distinct change of color may be termed alexandrite.
Like many other gemstones, alexandrite forms in a metamorphic environment. But unlike many others, its formation required specific geological conditions. The chemical elements beryllium (a major constituent in chrysoberyl) and chromium (the coloring agent in alexandrite) have contrasting chemical characteristics and do not as a rule occur together, usually being found in contrasting rock types. Not only has Nature brought these contrasting rock types into contact with each other, but a lack of the chemical element silica (the second most common element in the Earth's crust) is also required to prevent the growth of emerald. This geological scenario has occurred only rarely in the Earth's history and, as a result, alexandrite crystals are very scarce indeed.
Russia has remained the primary source of alexandrite since gems from the mines of the Urals became available on the market. In 1987, alexandrites were discovered in a place called Hematita in Minas Gerais, Brazil. The color of the Brazilian stones is admittedly not as strong a green as that of Russian alexandrite, but the color change is clearly discernible. Alexandrites are also obtained from sources in Sri Lanka, but the hue of these stones compares less than favorably with that of the Uralian alexandrites. They appear green in daylight and a brownish red in artificial light. The Tunduru area in southern Tanzania has also produced some outstanding specimens since the mid-1990s. Alexandrites are also found in India, Burma, Madagascar and Zimbabwe.
Adapted from http://www.gemstone.org/index.php?option=com_content&view=article&id=127:sapphire&catid=1:gem-by-gem&Itemid=14