The dark-green spots on these stones are the mineral olivine, or peridot. Olivine is special. Geophysicists believe olivine was the first mineral to form on a young Earth. Finding olivine-bearing rock where I live is extra-special. Olivine is igneous in origin. At the south end of Lake Michigan, bedrock is very deep sedimentary. No volcanoes or mountains around here for a very long time. This rock was brought to Lake Michigan’s Left Coast by glacier.
The olivine crystals on the larger of these rocks are quite worn and slightly depressed from the white quartz. They are scratched, sometimes extremely. There could be some water erosion. The rock is Precambrian; Precambrian rocks in Wisconsin vary in age from 925 million years to 3.5 billion years. These beauties are very old indeed.