History of Mani Stones
Mani Stones - often termed “magical jewels”—have long been believed to hold mystical power that aids in this divine manifestation. So the story goes, if you have a strong desire for a particular entity—be it something as precious and illusive as “treasure” or something as substantial and vital as food—you can come to acquire it with the help of mani stones.
To express their love of the faith, Buddhists began to mark stones with special images, phrases and mantras. It was a means of communicating their devotion, to the faith as a whole, as well as to specific deities. So, you see, these inscriptions are what differentiate mani stones from regular, non-sacred rock forms. What one chose to decorate the stone with, spoke to what was of value to the believer.
Popular images that were engraved are those of gods, buddhas, bodhisattvas, and impactful religious folk like monks and lamas. Writings that were sketched often consisted of a mix of more formal prayer and personal notes that reflected the engraver—how he was feeling, what he was working toward in his life, and what he was currently longing for.
Each mani stone will differ, for each inevitably is inscribed with unique engravings. The engravings selected reflect both the engraver’s current state, and his intention with the sacred stone. If he is looking, for instance, to create a mani stone that may be devoted to a deity upon its completion, all images and inscriptions will speak to that figure, and may even be planned in advance. If he is instead looking to offer up the artistic creation of his mani stone—meaning the process of inscribing the stone is a meditative act—the stone becomes a reflection of surrendering. Its creation becomes the ritual, rather than in the former case where the ritual takes place as the completed stone is offered up.
Source: https://www.tibettravel.org/tibetan-buddhism/tibetan-mani-stones.html