I've only seen it once, on a Kapp'n island. Never in my own town. But I generally don't hit rocks the full eight times any more, unless it's a money rock.
I actually think I got two gold nuggets once, maybe even three. I don't think I ever saved a replay or picture of it though sadly.
I haven't went on a Kapp'n trip in a long time, but I also got the shooting star island once. I was actually in the process of needing space themed diy to complete a room and thinking how am I going to craft all this stuff? I need a bunch of the zodiac fragments! And then as luck would have it, it was the first island trip lol.