A large mopane root. 
 
Mopane Roots are a popular decorative choice for many hobbiests and can be used in:
Fish Tanks 
Terrariums 
Vivariums 
Bonsai diplays 
Succulent Displays
 
Always wash the Mopane root before introducing it into your systems. Warm water and some scrubbing with a tooth brush will get rid of most unwanted particles.