Ahhh… Fresh from the bowels of a King Amoeboid.
— The Plumber receiving sewer crystals from Ratchet., UYA
Aquanos crystal

Ratchet standing near a Sewer Crystal

Sewer Crystals were found in the sewers of Aquatos. To get them, Ratchet needed to kill Amoeboid Kings or pick up crystals in the tunnels. The Plumber was looking for Sewer Crystals to make a necklace for his wife, so by bringing them to him, he would give 2,000 bolts for each one Ratchet brought him. In challenge mode, you get 20,000 bolts for each of them. There is little known about what made up the crystals, except that they were from the bowels of a King Amoeboid. The Plumber also said that he'd rather not tell you what they were, also hinting the origin may have been gruesome or gross. There were 101 crystals to find, 43 of which could only be found with the Gravity Boots.

Skill point

If you found all of them, you would achieve the skill point Hit the Motherload.