Its damage type depends on the dragon scale you have as a material component.
But how do GMs out there decide how many players can have? and what type?
Usually material components for wizards are negligible (at least, i never bother with them, unless their cost is important or particularly high); this is not the case!
I don't think you can just go into a dragon's lair and ask the owner if he has some spare scale "you don't happen to have some dried scale from last time you changed skin do you?"
It is important either what type of scale the caster has at hand when casting the spell, and if you allow the players to simply buy them,how much you charge?
Going by RAW, the spell is not listed as having an expensive component, so should be less then 1gp in cost. This would also fall under availability as part of a spell component pouch. (and Eschew materials)
House rules territory, its wide open but the spell is not exceptionally powerful that it needs a high cost deterrent/hard availability hurdle. A broken dragon scale could be found for sale in a given market anywhere, for a 1gp cost as a 'curiosity' value, for example.