Before revision: Several allotropes exist: amorphous boron is a brown powder; crystalline boron is silvery to black, extremely hard ( about 9.5 on the Mohs scale), and a poor electrical conductor at room temperature.
After revision: Several allotropes exist: amorphous boron is a brown powder; crystalline boron is silvery to black, extremely hard ( 9.3 on the Mohs scale), and a poor electrical conductor at room temperature.