The optical sign and birefringence of a cyclosilicate mineral are not only depended on the kinds of the ring of Si-O tetrahedrons and their orientations, but also on the crystallochemical sites of cations. These cations in cyclosi- licates may be classified into several types such as the large cations of "Mica- type", the octahedron cations of "Benitoite-type", the tetrahedron cations of "Beryl-type", the chain-type octahedron cations, the layer octahedron rations of "Sandwich-type" and so on. The one-dimentionality factor always co-exists with the two-dimentionality factor in a crystal structure. On condition that the one-dimentionality factor overpowers the two-dimentionality factor, the crystal has a positive birefringence, conversely, on condition that the two-dimentionality factor overpowers the one- dimentionality, the crystal has a negative birefringence. In the cyclosilicates the structure elements of Si-O tetrahedron ring are a two-dimentionality factor. The ca.tions of "Mica-type" and "Chain-type" are always a factor of one-dimentionality resulting in a positive birefringence, on the contrary, the rations of "Benitoite type" and "Sandwich type" are always a factor of two-dimentionality resulting in a negative birefringence. The cyclo- silicates with tetrahedrons of "Beryl-type" have a structure of three-dimentional framework, and have very low birefringence.