Japanese Art Motives by Maude Rex Allen 1917
"The Shippo or seven precious things are; gold,silver, coral, crystal,agate, and pearl. The list varies, and the older records include amber, tortoise-shell, and mother-of-pearl. The Japanese term for enamels of all kinds is Shippo; the word is probably of Chinese origin, and goes back to a time when amber, and tortoise-shell, mother-of-pearl, coral, agate, rock-crystal, and lapis lazuli were used as inlays. The shippo form is oval, like a flattened grain of rice, or like the small metal cells which, when filled with enamel, form the basis of much of the cloissonne' work. This simple form has been varied and developed into beautiful arabesque and diaper designs.
There is far more in this term shippo than can be grasped by us, because perhaps this mystic number seven is applied and made use of to express in its summing up what cannot be expressed by any other medium. Seven precious things to them are like the seven precious colors and the seven separate notes of music, by whose means all the glorious art of the world has found expression and interpretation."
Shippo BY THE WAY is given as one of the treasures filling the Takara-bune along with the seven lucky gods...