The smithing cap for this is either 80, 81, 82, 83, or 84.
Reasoning: I tried it at 79 and got a skillup, so it's greater than or equal to 80. But on that same skillup I broke the crystal. The only way to get a skillup on break is to be within 5 levels of the cap. That means it can't possibly cap higher than 84. Hence, the smithing cap is 80-84