Competent

For centuries mistaken for sapphire, because it did the work as well as sapphire did.

Blue Spinel · The Law of Potentiality

Family: Magnesium aluminium oxide — MgAl₂O₄
Hardness: 8 on the Mohs scale
Colour: Cobalt to greyish blue, no treatment needed
Signature: Singly refractive, brilliant, and famously untreated

The stone read as the word

Spinel sat in crowns for hundreds of years labelled as ruby and sapphire — the Black Prince's Ruby is a spinel. It was not a fraud; it simply performed at that level, and no one had the science to tell the difference.

Spinel is also one of the very few gems that reaches the market with no heating or filling. It is competent as found: tested, durable, and needing no help to be believed.

What it is carried for

Carried for ability that has been tried and held.
Held when she equips herself and takes the door she sees.