The stones need not match. Their materials and origins vary across the accounts; their small job is to mark where the cards have been placed. After a conversation wanders, the arrangement is still there to look at.
Position-based readers give them names: beginning, relation, consequence. Constellation readers object that the markers should not fix an order. The Emperor and Temperance belong to the disagreement, where a structure helps attention and relations may change what that structure reveals. The Oracle’s three cards do not acquire universal positions from these stones.
Imagine one rolling under a chair. For a moment the table looks incomplete. A superstition would make the missing stone invalidate the reading; the inventory explicitly rejects it. Someone can retrieve the marker, replace it or continue looking at the cards without it.
This widely attested object asks very little of its material. A stone can keep a place. It cannot decide whether what was said there was true.