| indestructible(adjective, firm, rigid, unyielding, steely) |
| tough(adjective, hard, firm, solid, stiff) |
| unbreakable(adjective, firm, solid, stiff, rigid) |
| adamantine(adjective, set, tough, firm, stiff) |
| rock-hard(adjective, firm, rigid, unyielding, steely) |
| flinty(adjective, firm, rigid, unyielding, steely) |
| impervious(adjective, set, hard, tough, solid) |
| compact(adjective, close, set, hard, tough) |
| obscure(adjective, dark, sophisticated, mysterious, complicated) |
| unfathomable(adjective, hard, knowledge, obscure, difficult) |
| incomprehensible(adjective, knowledge, obscure, solid, dark) |
| inscrutable(adjective, knowledge, obscure, blank, sophisticated) |
| dense(adjective, set, hard, tough, firm) |
| impassable(adjective, solid, dense, unaffected, unyielding) |
| impermeable(adjective, close, hard, tough, solid) |
| solid(adjective, close, set, hard, tough) |
| hard(adjective, tough, firm, solid, cruel) |
| opaque(adjective, set, solid, heavy, deep) |
| thick(adjective, dull, close, hard, tough) |
| arcane(adjective, obscure, solid, strange, doubtful) |