No, you cannot repair the Crystal Knight Shield or the Crystal Shield in Dark Souls. These shields are made of a special crystallized material that makes them extremely durable, but also brittle and unrepairable.
Crystal Shield
The Crystal Shield is a unique shield found in Dark Souls. It has the following properties:
Cannot be repaired
Parry is replaced by a shield bash
Blocks 100% poison, toxin and bleeding (does not work in PvP)
Has decent fire and lightning damage reduction
Requires 14 Strength to wield
The Crystal Shield is purchased from Domhnall of Zena for 4000 souls. It is considered one of the best shields in the game due to its high defenses and low stat requirements, but its inability to be repaired is a major drawback.
Crystal Knight Shield
The Crystal Knight Shield is similar to the Crystal Shield, but with slightly different stats. It has the following properties:
Durability reduced to 20
Base damage increased
Cannot be repaired (except through reinforcement)
Requires Crystal Reinforcement to upgrade
The Crystal Knight Shield is obtained by reinforcing a regular Knight Shield with Crystal Reinforcement. Like the Crystal Shield, it cannot be repaired once its durability is depleted.
In summary, while the Crystal Knight Shield and Crystal Shield are powerful and durable in Dark Souls, their inability to be repaired makes them a risky choice for extended use. Players must carefully manage their durability or risk being left with a useless shield.