Aventurine is a form of chalcedony, which in turn is a form of quartz. Aventurine is usually thought of as being specifically green - but it can also be white, grey, blue orange or other colors. Aventurine gets its color from inclusions of other minerals; and the typical green color that most people associate with aventurine, comes from inclusion of fuchsite , a chromium-containing mica, which has the delightfully complex chemical formula K(Al,Cr)2AlSi3O10(OH)2. 
L-R:Aventurine tumbled stone, rough, cabochon
Photo by Ra'ike - licensed under GNU 1.2
Quartz is the most abundant mineral on earth, and it comes in two forms - macrocrystalline, in which individual quartz crystals are at least large enough to be discernible with the naked eye, and cryptocrystalline, in which the individual crystals are only discernible under magnification.  Aventurine is composed of a cryptocrystalline quartz variety - and its glistening effect, caused by inclusions of other minerals, is known as aventurescence. 
In the past, aventurine was sometimes called Indian jade, although this is a misnomer - as aventurine is not a type of jade. The two minerals can be discerned apart by their specific gravity or refractive index. 
Much of the aventurine mined is used for carving and bead making, with a small number of the finest stones being cut into cabochons for jewelry-making. Some aventurine is also used in monumental architecture. 
It is known that Aventurine is sometimes treated with oil or dyed in order to enhance it's attractiveness to the consumer.  As always, buyer beware.
Aventurine is found in Brazil, India, Austria, Russia, Spain, Chile, and Tanzania; with the majority of the blue-green stone coming from India. Folklore maintains that the stone has a calming quality and brings fortune in games of chance (the name "aventurine" derives from the Italian phrase "a ventura", meaning "by chance" - and refers to the accidental discovery of aventurine glass in the 18th century).  
Note that aventurine should not be confused with sunstone, a completely different mineral which is sometimes also called aventurine feldspar.
Aventurine is relatively inexpensive and I have just seen (Jan 2011) aventurine rough for sale online for a few dollars a pound. 
Photo by Grzegorz Framski - released under Creative Commons 2.5 License
Aventurine - Sources Referenced:
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