Unless it is mixed with a lot of silver, most gold nuggets here on the east coast are 20-22K. If it's truly 10k then there are only two possibilities, there is a lot of silver in it or it is a melt the people who lived there did. Why, who knows?
Either way it's an awesome find and I've been told those acid test kits go bad very fast so you might be getting a false reading. I had that happen to me.
My best guess is that it's natural due to the shape.