T'ung Chih Trigram Coin Charm

In Chester L.Krause, Clifford Mishler, Colin R.Bruce II Standard Catalog of World Coins, Krause Publications, 22nd edition, 1995, we can read:

The eight trigrams (Pa Kua) of the Book of Changes (I Ching). The trigram is composed of combinations of pairs of eight trigrams each of which represents some power in nature, either active or passive, such as fire, water, thunder, earth, etc. These trigrams are said to have been invented 2000 years and more B.C. by the legendary monarch Fu Hsi, who copied them from the back of a tortoise.