Pearl is known as a protective stone and can be coloured to practically any colour. Pearls are products of bivalve mollusks (mainly oysters and mussels). They are built up of nacre (mother-of-pearl), which is mainly calcium carbonate in the form of aragonite crystals, and an organic horn substance known as conchiolin that binds the microcrystals concentrically around an irritant. Although pearls are not especially hard, only about 2.5 to 4.5 on the Mohs scale, they are extraordinarily compact and very difficult to crush. This necklace is made from mis-shaped pearls with small faceted ruby beads in between. This necklace measures approximately 18 inches in length with a sterling silver clasp.