arcane rainbow vellum

If an arcane caster casts a nondamaging arcane spell on the vellum, the vellum will generate a multicolored blotch resembling a Rorschach test.  The spread of colors identifies which schools of magic the caster knows and the relative size of the different colored parts provides a rough estimate as to how powerful the caster is in each school.
Black for necromancy, red for enchantment, orange for invocation, blue for abjuration, green for transmutation, yellow for divination, purple for conjuration, and shifting colors for illusion.
The vellum will not reveal which spells the caster has specifically though.
The vellum is very brittle so if a caster does not work measuring a mage who only knows invocation since invocation spells will obliterate the vellum.
The vellum is less informative with divine magic cast at the vellum always makes a silver blotch regardless of what divine spheres the caster has or who their divine patron is though stronger casters tend to create larger blotches.

History & Usage

History

Arcane rainbow vellum is a magical substance invented at the Uskalan Magic School that's use and manufacture has spread out across Scarterra, at least among places that like to regulate magic such Swynfaredia and Uskala.

Everyday use

If a nation can afford a steady supply of this vellum (it costs about 10 gold pieces a sheet), nations that register mages in their jurisdiction will make visiting and resident mages cast a spell on the vellum, then sign their name on the vellum. This gives the powers that be a vague idea of what the mages in their jurisdiction can do.
The Rorschach blot will generally be more geometrically regular for sorcerers than wizards, so sometimes this is used by Swynfaredians to screen out wizards pretending to be sorcerers but this is not very precise.
Very few nations bother to waste this expensive resource cataloging divine spell asters.

Distribution

Trade & Market

The raw materials to manufacture cost about 8 gold pieces per sheet. The creator needs to be able to cast Detect Magic and know how to make ordinary scrolls and have at least rudimentary skill in alchemy.

Type

Textile

Value

10-12 gold pieces a sheet