How does one get a chisel on Sims castaway DS?

For the chisels (one is metal and one is stone), you must need the Level 2 Crafting Bench (Bamboo Crafting Bench) or Level 3 Crafting Bench (Hardwood Crafting Bench). Just make them and you are done.

NOTE: You need the chisels to go to Geyser Rock. Hope this helped! ^_^