Surfaces that do not pull the eye
Unpolished silver returns no clear image, so there is nothing in it for the eye to catch on. Uncut stone is matte for the same reason: light goes in and does not come back shaped. A polished object holds the room. These hold their own colour and let the room go past.
Nothing that moves
Beads rattle. Charms swing. Clasps click against a band. If an object is meant to be worn while working, it has to be silent and it has to stay put. Every piece here is finished to sit still.
Clear, not busy
Much of what is sold as an aid to concentration is itself a source of detail — facets, inscriptions, moving parts. This axis goes the other way. The fewer decisions an object asks you to make about it, the less often it interrupts.