| impenetrable(adjective, firm, rigid, unyielding, steely) |
| indestructible(adjective, firm, rigid, unyielding, steely) |
| adamantine(adjective, firm, rigid, unyielding, steely) |
| flinty(adjective, firm, rigid, unyielding, steely) |
| tough(adjective, firm, rigid, unyielding, steely) |
| unbreakable(adjective, firm, rigid, unyielding, steely) |
| hard(adjective, firm, compact, rigid, dense) |
| rocky(adjective, firm, rigid, unyielding, steely) |
| steely(adjective, firm, rigid, unyielding) |
| stony(adjective, firm, rigid, unyielding, steely) |
| adamant(adjective, firm, rigid, unyielding, steely) |
| firm(adjective, hard, solid, compact, dense) |
| set(adjective, hard, compact, dense) |
| solid(adjective, hard, firm, compact, dense) |
| unyielding(adjective, hard, compact, dense) |
| brick wall(expression, adjective, hard, compact, dense) |
| close(adjective, hard, compact, dense) |
| compact(adjective, hard, dense) |
| compacted(adjective, hard, compact, dense) |
| concentrated(adjective, hard, compact, dense) |