Disclaimer: Tolkien assembled the world... Kortirion assembled the words.
Over many years it had been his habit to collect pebbles, small pieces of his belovéd Arda. Some rocks were for investigation; mornivren was honed into small blades sharper than steel in northern Rhun, while Greenstone became razor-edged adzes in the east... Some he picked up because the patterns pleased him; these he had fashioned into heart-shapes and gave to Arwen.
All except the iron-rich nodule found one memorable night in Moria; polished into a silvery oval, silken smooth, heavy in the hand... cool at first... but warming when cupped in his palm – this he kept privately, for himself.