What is your favorite geologic sample?

I don't have a whole lot of samples that I have discovered personally, but I am hoping to find some good ones this summer during my 210 class. However, I do have a pretty cool sample of basalt that I found in the Columbia River Basin about 10 years ago. It's very generic, but I was intrigued by the shape, the quantity of pores and the near weightless feeling of the tennis ball sized rock!