Calculative

Perfect metallic cubes stacked at right angles — a mineral that counts.

Galena · The Law of Position

Family: Lead sulphide — PbS
Hardness: 2.5 on the Mohs scale, very dense
Colour: Bright lead-grey metallic, cleaving into cubes
Signature: The first crystal radios were tuned with a galena detector

The stone read as the word

Galena breaks only along three perfect right angles, so it falls apart into cubes — the same shape at any scale. Early radio operators used it as a detector because it rectifies a signal cleanly.

Calculus is Latin for a small stone used for counting. This is that stone literally: geometry you can hold, arithmetic in a mineral.

What it is carried for

Carried when the plan must be costed, not felt.
Held for the number that decides which side a thing stands on.