With 75 Smithing, using 375 harmonic dust to make one at the crystal singing bowl in the Ithell Clan district of Prifddinas (giving 375 Smithing experience). You need to have completed The Eyes of Glouphrie quest.
The amount of coins needed to create (and repair) is based on the number of times a crystal weapon/armour seed has been made into an item for coins - the cost is shared over all crystal weapons and shields. The first time costs 1,000,000 coins, and each subsequent creation costs 200,000 less until the fifth time (and every time thereafter), when it costs 200,000 coins.
The crystal dagger will revert to seed after 50,000 charges of combat, but uses two charges per hit except while within Prifddinas, where it uses one charge per hit. A partially degraded dagger can be recharged for a fraction of your current creation cost. For example, if it has 90% charges remaining, and the lowest creation price has been achieved, it would cost 20,000 coins to repair.
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