| adamantine(adjective, tough, firm, stiff) |
| impervious(adjective, hard, tough, solid) |
| compact(adjective, close, hard, tough) |
| dense(adjective, hard, tough, firm) |
| solid(adjective, close, hard, tough) |
| opaque(adjective, solid, heavy, deep) |
| firm(adjective, close, hard, solid) |
| deep(adjective, obscure, solid, heavy) |
| heavy(adjective, hard, tough, solid) |
| close(adjective, hard, tough, solid) |
| condensed(adjective, hard, tough, solid) |
| compressed(adjective, hard, tough, firm) |
| rocky(adjective, firm, stiff, compact) |
| packed(adjective, close, hard, firm) |
| substantial(adjective, close, strong, hard) |
| unyielding(adjective, hard, tough, firm) |
| concrete(adjective, hard, tough, solid) |
| crowded(adjective, close, solid, heavy) |
| inflexible(adjective, strong, hard, tough) |
| rigid(adjective, strong, hard, tough) |