If one were to make ices before Yom Tov (basically a big peice of frozen juice), and before serving the ices one were to take the spoon rub it against the ice in order to make it more of a sherbert, so the ice now takes the form of little tiny pieces, is this a problem to do on Yom Tov because this could have been done beforehand? Is this a problem to do on a normal Shabbos? If it is allowed is it only allowed immiediatly before eating or can it be done whenever?
Although Shulchan Aruch (320:9) writes that it is forbidden to grind up snow and ice on Shabbos, this is only true becasue of the change of state (from solid to liquid) that grinding the ice causes (this is implied by the wording of Shulchan Aruch, who writes that it is forbidden to break the ice into small pieces so that its water should flow; see also Taz and Pri Megadim, 320; Mishnah Berurah 318:102).
If the intention is only to change the texture of the ice while leaving it in its frozen state, rubbing the spoon against the ice would involve no prohibition on Shabbos or on Yom Tov, and would be permitted whenver, provided it is being done for the purpose of consumption on Shabbos (or Yom Tov, such that no prohibition of hachanah is involved). However, this is only true insofar as the ice is eaten as ice (solid), and not as liquid, and if the rubbing and grinding would involve even a partial transformation of the ice into liquid, it would not be permitted.