Those stones Amakusa stones come from a quarry on the island Shimoshima, in the prefecture of Kumamoto.  They are a beautiful natural stone and very fine.

This item from Amakusa has two types the red color and comes with #500/#800 grit and the other one is white color that comes with #1000/#2000 grit

Red Amakusa

Length:205 mm
Height:75 mm
Width: 43mm
Weight: around 2KG

White Amakusa

Length:200 mm
Height:70 mm
Width: 40mm
Weight: around 1.2KG