長手甲 will produce an ugly sound when you play, but otherwise will do no damage on the keyboard.
琴鍵, the keyboard, is now made of very durable Ivorite (invented by Yamaha). It's not quite possible to 弄花 Ivorite, unless they are defective. I cracked some Ivorites on a Yamaha before, but from over-use, not from scatching.
However, if you have a very old piano that uses ivory instead, then it might be possible.
2007-01-23 07:33:12 補充:
Sorry, really hard to believe. Your fingernails harder than Ivorite keys?