We usually use not signature but seal when we sign contracts in Japan.
A boxwood trees, ivory, some sorts of precious stones like jade and crystal are used for the materials of the seals. A seal made of acrylic resin and made of titanium is also popular recently .I like seals that are made by precious stones.
Ivory was forbidden to import, but now, a constant quantity of ivory is imported under government control.
It is said that the design of the sculpture of the seal have the power to change the owner's fortune. Some people try to sell seals unreasonably high price by using this folk belief for the wrong purpose.