| unbreakable(adjective, strong, tough, firm, rigid) |
| impenetrable(adjective, firm, rigid, unyielding, steely) |
| permanent(adjective, strong, tough, sturdy, constant) |
| tough(adjective, firm, rigid, unyielding, steely) |
| adamantine(adjective, firm, rigid, unyielding, steely) |
| rock-hard(adjective, firm, rigid, unyielding, steely) |
| flinty(adjective, firm, rigid, unyielding, steely) |
| eternal(adjective, pause, endless) |
| everlasting(adjective, strong, stable, constant, unable) |
| durable(adjective, strong, tough, stable, sturdy) |
| imperishable(adjective, moral, constant, stability, unable) |
| enduring(adjective, tough, stable, sturdy, constant) |
| lasting(adjective, strong, tough, stable, sturdy) |
| fixed(adjective, tough, stable, sturdy, constant) |
| perpetual(adjective, stable, moral, constant, unable) |
| immortal(adjective, unable, lasting, imperishable) |
| abiding(adjective, tough, stable, sturdy, constant) |
| constant(adjective, tough, stable, sturdy, enduring) |
| immutable(adjective, stable, constant, enduring, unable) |
| undying(adjective, stability, unable, lasting) |