18 inch necklace. Deborah Blake is a founding member of the Artisan's Guild in Oneonta, N.Y. and makes handcrafted gemstone jewelry. In this piece she uses Rhodonite, a mineral liable to alteration, with the formation of manganese carbonate, hydrous silicates, or oxides. The compact material, which is cut and polished for ornamental purposes, is often marked in a striking manner by veins and patches of these black alteration products. You can see this in the triangular pieces used in this necklace.