Crystals are a type of item in Dark Chronicle. They are used for synthesis to increase a weapon's stats, as well as to invent items. The elemental Crystals are visually identical to the Attachments used in Dark Cloud. Although most other items can also be spectrumized, crystals are the only (commonly) available item that will raise a stat by three, excluding weapon Synth Spheres and gemstones. The Power and Protector Crystals can also be substituted for other items, notably the Knight Boots for the former, as Power Crystals are relatively rare.
Regardless, other Crystals are fairly common, typically found in treasure chests, dropped by or stolen from an enemy. They are sometimes also available as a reward for completing Spheda or winning a Fishing Contest or Finny Frenzy. Rufio sells the four elemental types once he is recruited through Rufio's Elements, with the entire set being available in Chapter 3 at Starlight Items. In Chapter 8, when the future becomes inaccessible, Rufio will sell the entire range of Crystals available. Crystals have a carry limit of 999, due to how common they are.