General synonyms:
* Rock
* Boulder
* Pebble
* Gravel
* Gem
* Mineral
More specific synonyms:
* For building material: Brick, concrete, marble, granite
* For a precious stone: Jewel, gem, diamond, ruby
* For a small stone: Pebble, gravel, grit
* For a heavy object: Weight, ballast, anchor
Figurative synonyms:
* For a hard heart: Heart of stone, unfeeling
* For a difficult task: Hard nut to crack, tough cookie
* For something unchanging: Unmovable, steadfast, resolute
The best synonym to use will depend on the specific meaning you want to convey.