Mineral site, and from there further subdivided by the chemistry of the W site. The Y and Z sites can accommodate a variety of cations, especially various transition metals; this variability in structural transition metal content gives the tourmaline group greater variability in colour. Other cyclosilicates include beryl, AlBeSiO, whose varieties include the gemstones emerald (green) and aquamarine (bluish). Cordierite is structurally similar to beryl, and is a common metamorphic mineral. Sorosilicates, also termed disilicates, have tetrahedron-tetrahedron bonding at one oxygen, which results in a 2:7 ratio of silicon to oxygen. The resultant common structural element is the [SiO] group. The
Prong setting shapes, depending on the shape of the gemstone, the size of the gemstone, or whether or not the gemstone is faceted or cabochon shaped. Common shapes of heads used in stone setting include round, oval, emerald cut, princess cut, marquise cut, pear shaped and trillian cut. V-shaped prongs are recommended for a marquise-cut or pear-shaped stone, while flat prongs are recommended for a flat-cut emerald. A prong setting holds the gem securely while allowing the gem to be struck with more light and from more angles, increasing the brilliance of the gemstone. A prong setting offers less protection to the