Corrective

The abrasive that every other stone is trued against — hardness that corrects.

Corundum · The Law of Perfecting

Family: Corundum — aluminium oxide; sapphire and ruby are its gem forms
Hardness: 9 on the Mohs scale
Colour: Blue with straight, hexagonal growth banding
Signature: Emery — crushed corundum — is the world's oldest polishing abrasive

The stone read as the word

Before it is a sapphire, corundum is the grit on a sharpening wheel. Its industrial name, emery, is what generations of craftsmen used to bring metal and stone to plumb.

Corrective work needs something harder than the thing being fixed. Nine on a ten-point scale, and every growth line in it is straight.

What it is carried for

Carried when she must be the one who trues the work.
Held for correction given straight and without malice.