Volcanic ash will vary in size or grit. Depending on how far from ground zero it will be sorted by natural forces like the wind or sometimes water. So depending on where they got it one persons ash might be bigger than another.
Volcanic ash will also vary in hardness and sharpness depending on the nature of the rock type being ejected and the type of eruption you are dealing with.
That being said it is possible to check all that but probably a bigger pain than buying a known size grit as in Chromium Oxide.