A matter transmuter was a device that could change the state or type of matter.
Before the end of the 22nd century invasion of Earth, the Daleks developed a matter transmuter that could change any element into any other. The Master planned to steal it and use it to extort whole civilisations. He was stopped by Susan Foreman, who shot him with his own Tissue Compression Eliminator while he was holding the transmuter. (PROSE: Legacy of the Daleks)
Jebelex the Pulthasian had a matter transmuter that could change solids to liquids. He used it to steal a Treed Crystal from Honey Vox by liquifying the wall of the safe where the crystal was stored. (COMIC: Jewel of the Vile) After Jebelex was killed, Martha Jones used the transmuter to solidify the liquid creature that had consumed him and the crystal. The Tenth Doctor then shattered the creature and retrieved the crystal. (COMIC: Lock, Stocks and Barrel)