| heart of oak(expression, idiom, strength, hardness) |
| heartless(adjective, mean, hard, tough, harsh) |
| hostile(adjective, person, feelings, response, emotions) |
| impassive(adjective, cold, indifferent, steady, fixed) |
| indomitable(adjective, strong, hard, tough, determined) |
| insensitive(adjective, difficult, cold, indifferent, troubled) |
| insubordinate(adjective, tough, firm, unreasonable, grim) |
| invariable |
| lithic |
| marble |
| nail |
| peremptory(adjective, opinion, description, characterization, judgement) |
| petrified |
| pig-headed(adjective, determined, resolute, obstinate, uncompromising) |
| purposeful(adjective, firm, deliberate, determined, resolute) |
| rebellious(adjective, tough, firm, unreasonable, grim) |
| rock-ribbed(adjective, determined, resolute, uncompromising, unshakeable) |
| rough(adjective, person, strong, hard, tough) |
| rugged(adjective, strong, hard, tough, sturdy) |
| stoic(adjective, cold, indifferent, unyielding) |