Several allotropes of boron exist : amorphous boron is a brown powder ; whereas crystalline boron is black , extremely hard -LRB- NUMBER@1 on PERSON@1 ' scale -RRB- , and a poor conductor at room temperature .

Amorphous boron is a brown powder and metallic -LRB- crystalline -RRB- boron is black and hard and a weak conductor at room temperature .
