Speaking of a kalpa, suppose there is a huge blue agate, an 80,000 ri cube, which does not erode even if it were filed for aeons. Suppose an angel descends once in three years to caress it with her extremely beautiful and light robe. The length of time required for the angel to wear out the blue agate is referred to as a kalpa.

Matsuno-dono Goshōsoku, Letter to Lord Matsuno, Writings of Nichiren Shōnin, Volume 7, Followers II, Page 66