Wikimedia
Wikipedia: Jasper From Wikipedia
This site has a concise definition of jasper along with pictures and connections to related sites.
Jewelry Central
Jewelry Central: Agate
A commercial site that contains a brief paragraph about agates and photographs of some varieties.
Other
Bob's Rock Shop: Rock & Gem: Agates From the Land of Pumas and Craters
An article about hunting agate in Argentina, from Rock & Gem magazine.