To obtain the resources that a Boulder holds, you must first have the appropriate tool, a Pickaxe. Although a Stone Axe gets the job done as well, it is less effective and therefore should only be used in the early game.
A boulder has three stages of destruction, each stage yielding another round of loot.
Stage 1
The Boulder is simply that, just a huge rock!
Stage 2
The Boulder has been broken once, it should now be in two separate pieces.
Stage 3
The Boulder is now a single one of these pieces. After destroying it, the Boulder will be gone.
Note: There are multiple models for each of these stages.
The amount of loot as of Version 15 (possibly earlier) no longer depends on the quality of the tools used or of skill of the player. Each stage gives the following: