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 the Dragon warhammer and use the "Smash" option on the crab. This will bind the crab on the spot for a while.