This is a stone I found in the Bumthang Valley river, central Bhutan. The engraving is of a double vajra with the mantra of the Bodhisattva Manjushri above.
Manjushri represents the embodiment of the knowledge and wisdom of all the Buddhas. He made the aspiration to come back again and again in this world until every sentient being is totally free from suffering. The stone was made for a teacher who is known in Bhutan as an emanation of Manjushri.
The double vajra has numerous meanings, but basically represents unshakable indestructibility.