Here are the most important elements which are used to build Boulder Dash caves:
Rockford: Your hero. You have to guide him through the caves, searching for diamonds.
Empty Space: Just empty.
Dirt: You can dig through this dirt with Rockford. However, most of the other elements cannot move through dirt.
Boulder: A big rock which can easily block your way. A boulder falls down when there's emty space under it. So watch out when there are boulders above you; you'll lose a life when crushed! Furthermore it's possible to move a boulder by pushing it with Rockford.
Diamond: Very important! To complete a cave you have to find a certain amount of diamonds.
Wall: Sometimes you can't get some diamonds, because they're hidden behind a wall. Fortunately it is possible in some way to blast through these walls.
Titanium Wall: Also a wall, but an indestructive one. You cannot destroy it by explosion.
Magic Wall: This very special wall converts boulders into diamonds and vice versa. Note that a magic wall can only be activated for some limited time.
Firefly: This is one of your enemies. You'll die when you touch it, but you can also attack the firefly by yourself! It explodes when you drop a boulder on it. All the elements surrounding the firefly (except a titanium wall) will be destroyed too. You can use this, for example, to blast through an undestructive wall. Fireflies move in a predictable clockwise pattern along the border of empty spaces.
Butterfly: The other animal. Like the firefly, the butterfly is deadly on touch. The most important difference, however, is that you get 9 diamons when a butterfly explodes! Butterflies move counterclockwise along the border of empty spaces.
Amoeba: A sort of slimy mass, which can grow and grow up to 200 units. Then it will turn into boulders and you'll get a lot of boulders in this case. But if you enclose the amoeba by boulders or other massive elements, then it will turn into diamonds. Fireflies and butterflies immediately explode when touching the amoeba. This gives a nice explosion effect when you send a long row of butterflies to the amoeba!
Exit: This exit will be revealed when you have collected the required number of diamonds. Now you can enter it and you are transported to the next cave, after getting some bonuspoints.