An important, rich old specimen of the mineral devilline (formerly ‘herrengrundite’) dating to the world-class finds of the mid-1800s in present-day Slovakia. This piece consists of striking, deep teal colored rosettes comprised of 1 to 4 mm, platy crystals densely covering one side of the matrix. Devilline is a rare hydroxylated calcium-copper hydrate. Specimens of this size and quality rarely come up for sale. This is a special, displayable rarity in good size, and has a unique color.