The mountain behind Sukhothai on the west was called Khao Phrabat Yai, located on the edge of the ancient water reservoir called "Saridphong". On the top of the mountain, there was a historic site with the Buddha's footprint built in the reign of King Maha Dharmaracha Lithai and there was a sacred place called "Khao Tok Phra Ruang (Phra Ruang Popped Rice)" scattered on the ground.

It is said that when Phra Ruang was eating rice, he spilled rice on the floor and bewitched it to remain afterward and those possessing it would be lucky. Popped rice became a cube object with black, brown, or rust, buried in soil and rock layers until the present.

This story took place to describe a mineral called LIMONITE or PYRITE, a type of iron oxide with natural cube shapes. It usually appears in the metamorphic layers found on Khao Phrabat Yai. Nowadays, this kind of mineral is cut and polished to make it look black and shiny for producing fortune accessories.