Rock Crabs are accessed through the Training teleport > Rock Crabs. They are a good place to start after you've bought starter supplies at the Edgeville General Store. Typically there is not a lot of players there, and it's a great place to start on working on combat stats as well as Magic and Ranged.
Blue Charms - 0.5% Chance.
Green Charms - 4.166% Chance.
Gold Charm - 3.03% Chance.
Crimson Charm - 14.28% Chance.
Note: The rest of the drops are not worth picking up.