Note to self: I should probably know for sure what something it is before I post it for this week's challenge of "What is this?" Nah, that might not happen!
You had such good guesses yesterday! And I think Becky's is the most accurate. This is a larger, more distant, view of this polished agate/rock thingy. I don't exactly know what it is, and have wondered if it does contain some kind of fossil in it. But I think it's a slice of agate, polished. It may not be clear from this photo, but the bottom is flat, and the top is more dome-shaped.
I'm pretty sure you'll figure out what this is: