The seal, a longitudinal one in green Jade, represents a roaring imperial lion with the mouth towards the sky. The base of the seal has the four small seal script characters (as you can see it has a fissure). It seems to be from the Empress Wu Zetian (690-705).
I did my research and find out the following: 子古杀天, Zi gǔ shā tiān, Emperor Wu Zetian
古 Antiquity 天 Sky 子 Son 杀 Kill Gǔ zi tiān tiáo
Please find below the seal mirror image:
Many Thanks in advance.