It looks to be common quartz. The staining may be impurities or it might be limonite, which is an iron trace found in many Great Lakes rocks. Darker quartz, purple in color is amethyst, reddish tones are called rosy quartz, dark tones, smoky quartz. All are found commonly in the Great Lakes area. This specimen in my hand has agate, white quartz and in the center amethyst. They are only worth what someone will pay. Yours is very large. Some might want to carve it into something so it may have some worth. It's cool. I have some from Lake Superior which are fully smoothed into egg-shapes as large as my fist. We welcome all inqiries, especially with pictures and source information.