User blog comment:Zephyrcoil/RWBY Volume 2: Commentaries and Other Information/@comment-25258408-20141128034421/@comment-4010415-20141128180857

This Tumblr post pretty much explains exactly what I mean that Weiss' semblance is glyphs, not ice, with screenshots and whatnot.

We've actually apparently seen Weiss use ice dust with her semblance before, all the way back in volume 1. In the screenshot below, notice the glyph on the ground. Weiss could have used her glyph to influence what shape the ice would take.