I hadn't heard of thundereggs before I started researching for a trip around Australia. My husband and I visited Mount Hay where we could fossick through mounds of dirt. We did a tour, saw how they polished gems and worked in pewter and after we found our thunderstone they cut them in half for us. In all a great experience.
Yes I have heard of them, and even seen some.
I saw them in a shopping centre for sale on a stall.
The one in my pic is what I saw (type) and are very lovely.
Not all are as nice, but some are quite interesting,.