| indestructible(adjective, strong, tough, firm, rigid) |
| tough(adjective, strong, firm, solid, stiff) |
| adamantine(adjective, strong, tough, severe, firm) |
| rock-hard(adjective, firm, rigid, unyielding, steely) |
| flinty(adjective, strong, hard, firm, solid) |
| impenetrable(adjective, firm, solid, stiff, rigid) |
| shatterproof(adjective, strong, tough) |
| infrangible(adjective, strong, tough) |
| unyielding(adjective, strong, hard, tough, severe) |
| rigid(adjective, strong, hard, severe, firm) |
| firm(adjective, strong, hard, tough, severe) |
| steely(adjective, firm, rigid, unyielding) |
| solid(adjective, strong, hard, tough, severe) |
| hard(adjective, strong, severe, firm, harsh) |
| resistant(adjective, strong, hard, tough, solid) |
| adamant(adjective, firm, rigid, unyielding, steely) |
| rocky(adjective, firm, stiff, rigid, unyielding) |
| rugged(adjective, strong, tough) |
| sturdy |
| tight(adjective, strong, hard, tough, solid) |