a glass - ceramic which is substantially and desirably totally transparent , and which contains a predominant crystal phase of forsterite . the glass - ceramic is formed from precursor glasses having the following compositions , in weight percent on an oxide basis : sio 2 30 - 60 ; al 2 o 3 10 - 25 ; mgo 13 - 30 ; k 2 o 8 - 20 ; tio 2 0 - 10 ; and geo 2 0 - 25 . the glass - ceramic may be doped with up to 1 wt . % chromium oxide to impart optical activity thereto .