I found this rock in Lake Michigan, Michigan side. It is rather soft (softer than a Petoskey I would guess) and light. The stand is the natural rock pretty much looks as found. Sphere is made from same rock. Polishes to a matte finish Any ideas?
Cant say as I can make a good guess with that one Rattlehead. The polished sphere has a layered look that suggests sedimentary rock and the base bit is fairly uniform. Possibly vitrified sediment. Harder than chalk but not dense enough to be considered a solid silicate. Maybe someone else can pin it down. That area of the country is known for some unusual stuff due to glacial deposits. Nice work.
I have to agree with DDancer on the composition of the rock as I have seen somewhat similar specimens, although they were usually in slabs! Due to the softness of the material and not being able to obtain a high gloss when polishing it, have you thought about stabilizing it in the same manner that Turquoise is stablized? I believe that if the stabilizing process works on your' stones, you will end up with a great result. In that, I mean that the material will be hardened and more susceptible to a high gloss polished finish which will compliment some of the other beautiful pieces that you have made.
If you find a piece your not to afraid to test, do an test with acid. See if it starts to bubble and fizzle.
Check under a loupe to see how the polished surface looks, do "grains" miss, as in forming small pits?