The world: A standard medieval European-ish fantasy world, with very limited magic, which aside from the titular exception has little to no effect on everyday life. The continent is divided into around a 60 petty kingdoms of about the same military power and technological level as Viking Age kingdoms had.
The scenario: while magic in the world usually proved underwhelming, under-powered and in general no more useful than its non-magical equivalent, one spell became surprisingly effective: Companion Rock Enchantment:
- every human being is capable of casting this enchantment, as long as they talked to someone else's Rock beforehand (it takes a long, spiritual conversation with someone's Rock that "unlocks" their potential). Rocks are very reluctant to talk to anyone other than their Owner, unless the owner explicitly orders them to.
- All it takes to create a Companion Rock is taking a fist-sized stone, carving eyes and a mouth onto it, and giving it a kiss.
- resulting Companion is an immobile rock that has a mind as intelligent, knowledgeable and capable as its Owner at the moment of creation, but can learn. Its memory is not perfect, but superior to human due to better mental focus and not being easily distracted. It can speak, see, hear and smell as well as a human would even though it does not have organs to do so. The Rock is 100% devoted and loyal to its Owner. Being a piece of stone, it has no real emotions, but can pretend to emote if it pleases the Owner. Otherwise it remains stoic and relentlessly rational, in its pursuit of its goal: helping the Owner be safe, happy, and fulfilled. Otherwise, the personality of the Rock tends to be similar to its Owner, except more stoic, practical and rational.
- A person can only have one Rock at a time. Enchanting a new one simply kills the old one.
- Rocks that outlived their Owners bond with the next person that touches them, as long as it is someone without a Rock of their own.
- otherwise, the Rock ownership cannot be sold, inherited, leased etc: either you enchant your own, or find an ownerless Rock.
The Question: how would the proliferation of the Rocks affect the economy? Lets assume that the economical situation when the Rock Enchantment is discovered is subsistence-level farming + rudimentary feudalism, and a vast, complex, but relatively low-volume continent-spanning trade. My question is mostly about economy/wealth generation, but if the answer requires delving into technological development, culture, etc this is more than welcome. Assume there are no religious or magical barriers to people using the Rocks to get rich and improve their lives, the only limits are their imagination and competition from others.