I'm pretty generous with my "stuff" - I lend and give away things often. For me...I'd probably just lend her the pattern unless I believed I'd use it again and she wasn't likely to return it promptly and in good condition. In which case I'd just say that I don't want to lend it out until I'm done with it, and I'm not done with it yet.
But it doesn't matter, that's what *I* would do. You say you don't want to lend the pattern out...so don't! Just tell your coworker your honest reason, whatever it is. (Unless maybe it's something like, "you have bad body odor and I don't want my pattern coming back stinky"...maybe find a tactful excuse if it's that sort of situation, lol)
It's perfectly OK to say no to people. If you do it honestly, MOST people will respect that and appreciate your honesty. The ones that don't...it's probably not a bad thing if they avoid you a bit in the future.