They are extremely simple to operate. All you do to activate them is focus on seeking something and drop one or more stones. They will then roll via what superficially looks like coincidental or terrain-based momentum towards what is sought. This is all that was revealed by the previous owner before he either dies an unnatural death without being murdered (in some way that means he disobeyed a geas) or kills himself (likely same).
The complication is that the apparent intelligence of the pathfinding increases with the number of stones involved.
A solitary stone rolls at a walking pace directly towards what is sought, irrespective of obstacles, hazards, or the speed of movement of the user. It will roll straight into canyons, fire, etc., and if it hits an obstacle more than double its height, it just stops. If not obstructed, it will roll whether or not it is still being followed, and can be easily lost.
They get "smarter" the more you drop, avoiding obstacles and hazards, stopping or speeding up dependent on the speed of movement of the user, etc. All of this can be revealed in test logs.
I am also thinking that, while they appear to just be made of standard mid-quality quartz of a particular variety comparable to quartz found somewhere in Norway, they are very, very hard to break and/or have a self-repair ability. I want there to be every reason to believe that all 13 are still in existence, albeit possibly in very hard to reach locations.
The worrying part is, there are definitely more not in Foundation control. The marbles came in a leather bag that had slots for each stone. Twelve slots, plus a space in the middle for a larger sphere. Two of the slots are long-crumpled, their stones missing for a long while. Three more look more recently used. During tests, it is established that the stones can be used to look for their fellows, and that they seem unphased by various Foundation standard methods of obfuscation. Question is, who has the others, and how many are held in a single entity's possession?
There are also possibilities for revealing secrets even if you don't follow the stones the entire way (they are reasonably easy to stop partway, especially if you drop several). While they will only lead to something that could be walked to, they don't distinguish between items that can be reached by mundane methods and those that can only be reached through various types of anomalous portals. A secret space station has been mentioned as reachable via a terrestrial SCP. If asked to seek "space" or "low Earth orbit" or wherever that station is, they could lead straight to that skip. Putting in a few experiments where researchers try to stump the stones but get a response could be great for story hooks.
And finally, there is that larger stone in the middle. I have an idea for what that does, but I am not firm on it yet. Need to do some more boning up on my Norse mythology before I am sure.