Alchemy

Alchemy (錬金, Renkin), also known as transmutation, is a basic spell in earth magic, which enables a mage to convert any material into another material or substance according to his or her choice. As an example, Chevreuse was able to convert three pebbles into brass.

Variations
It is unknown up to what extent an earth mage could convert a substance into another, let alone if a pebble or rock is the only thing that the spell could originate from. As observed when Louise de la Vallière attempted to demonstrate the spell, a light with various visible spectra appeared to shine prior to converting pebbles (albeit unsuccessfully, as it only resulted in an explosion), which may indicate or hint at the mage's choice and preference of the resulting product. Moreover, Jacques was able to convert a rock into a weapon of his choice, specifically a flail, which he used in his combat with Saito Hiraga.