WordNet sense 1 (caramelized sugar cooled in thin sheets):

toffee, toffy

WordNet sense 2 (lacking in sympathy or feeling):

coldhearted

WordNet sense 3 ((of metal or glass) not annealed and consequently easily cracked or fractured):

unannealed

WordNet sense 4 (not brought to a proper consistency or hardness):

unhardened, untempered

WordNet sense 5 (having little elasticity; hence easily cracked or fractured or snapped):

brickle, brickly

WordNet sense 6 (capable of being broken or damaged):

breakable