Quote from casualgameguides.com:
There is a crossbow contraption in the center. You can use it at anytime on the large blue and orange crystals (and later a green one); however, you cannot break through the large crystal blocking the way.
You use the top two buttons to turn the crossbow.
You use the bottom two buttons to raise and lower the angle.
Pressing on the crossbow itself will let an "arrow" loose.
You need the hit the large crystal 3 times and it will explode and leave gems.
http://www.casualgameguides.com/games/guide/game_guide.cfm/Totem_Tribe_Gold_Extended_Edition-Walkthrough/Cheats-1/game_id-9958/#crash has image to help.