These are the remains of Yayoi pottery cup pedestals and high cups. There are a variety of pieces, from those with hammered marks to those containing mica.
How about using it as a reference material or an object?
First photo (from left): w10.5 x d10.2 x h5.5cm, w9.5 x d9.5 x h8cm, w7.5 x d8 x h4.5cm