Two Chinese characters Wu Zhu // Inner and outer rims. 36mm at the longest, the coin is 25mm or so, 5.43 grams. Hartill -; Gratzer/Fishman #B3.1. 

Interesting mint error - the coin tree was miscast - the incomplete coin with an attached part of the adjacent coin was discarded. Extremely rare error - most such coins were melted and recast. Unique, the only such coin I've seen.

These Wu Zhus were cast during the brief reign of the unfortunate Emperor Gengshi, a member of the Western Han dynasty to briefly ruled the Empire after the fall of Wang Mang in 23 CE.  The Gengshi Emperor was viewed as a weak and incompetent ruler who was unable to keep that empire together. After a reign of two years he was deposed by the Chimei ("Red Eyebrows") rebels and strangled a few months after his defeat.

They are fairly scarce in general (rated “Rare” in Fishman/Gratzer), and very rare in extremely high quality like this piece.