Unfired clay or ceramic? spindle whorl

Spindle-whorl, diam 3.2cm. Thick disc-shaped, with rounded edges. Top decorated by 3 concentric grooves; faint grooves 4 around side, and also 4 concentric grooves fainter towards edge, around base. The material is odd! Dark brown with red-brown interior when surface nicked. At first it seemed a strange amber, with some electrical properties of amber, but not so strong. But it is not amber. Neither is it stone. It might possibly be a curious fine red clay, without grits - there are slight surface cracks which support this.Cf. Sibertswold M 6560 (Unknown grave) for similar fabric (clay!!).