Certain forms of the igneous rock syenite are nicknamed emberlite because they contain sodalite, causing them to fluoresce under black light. Glaciers deposited lots of them in the Lake Superior basin, where rock hunters go out at night to find them.
Here's a nice how-to video on finding them, from Erik Rintamaki, a tour guide for rock hunters:
Yooperlite is a trade name for the rocks (people from the upper peninsula of Michigan are called Yoopers), and Rintamaki apparently enforces the trademark, causing a lot of people to call them emberlites instead. Here are some more fine examples of these beauties:
These guys get a pretty good haul, too:
Image: YouTube / The Crystal Collector
