Miwok Indian clay marble

I live in Foresthill, CA., in the heart of Gold Rush country. I's know that the Miwoks were very active in our area before the miners took over, more or less. There are numerous gringing stones that date back 3000 years.

I found this marble buried in the dirt next to my house and checked with the Placer County Historical Society about it's authenticity. The unusual aspect of the marble is the coloring. Most were greyish to light brown. As you can see, this one is mottled with green. I can't explain this and wondered if anyone out there has run across anything similar.

Diameter is 1.25"

mmudge@ftcnet.net