The Oblong Crystal is made from a stone found on Lorien, one such crystal was brought to Earth in Four's Loric Chest.

It was a perfectly smooth oblong, clear on the outside and cloudy in the center. The crystal is only active when the person holding it is developing legacies, hence it begins to heat and the cloudy mass begins to swirl when Four holds it, his Lumen in both hands are also activated.

Shortly after Four develops Lumen, Henri uses the crystal over his skin and the heat resistance in his hands are slowly spread to the rest of his body.