"Manju" Stone

• “Manju” Stone
  Kurayoshi-shi, Tottori-ken, Japan
  
  When you break one of these stones, which are called Isshiki-mame in Tottori region, you will find dark-brown center. They have been found around Daisen area, Tottori-ken since Edo Period. Many are found in strata forming hills.
  Pumice stone scattered on the occasion of Daisen’s volcanic activity contained black mica. This black mica was decomposed and made iron and manganese substances to make the core. Around this core agglutinated alumina-rich gibbsite, and formed pea-like “Manju” Stone.