Jewelled Crabs are found in the crab puzzle room within the Chambers of Xeric. These crabs cannot be killed by conventional combat, however the crystals which are on the crabs can be attacked in order to change their colour. Attacking with melee (or smashing it with a hammer) will cause a crab to change Red, magic for Blue, and ranged for Green.
For solo players, they can use a standard hammer, or any blunt weapon with crush bonuses, including but not limited to: the (dragon) warhammer(s), the elder maul, the abyssal bludgeon, the cursed goblin hammer, Torag's hammers, granite maul, granite hammer, frying pan, meat tenderiser, etc. and use the right-click "Smash" option on the crab. This will stun the crab in place for a while.
Each crystal requires the following orbs: