Bulk glass having a calcium-mica composition (Ca 0.5 Mg 3 AlSi 3 O 10 F 2 ) is homogeneous glass. The crystallization mechanism of the mica is surface crystallization and transparency is lost completely when crystallization occurs on the surface. In this study, by decreasing SiO 2 and increasing CaO and Al 2 O 3 from the chemical composition of Ca 0.5 Mg 3 AlSi 3 O 10 F 2 , and moreover by replacing a small amount of K 2 O instead of CaO, the phase separation appears in the glasses. Because of this phase separation, the mica begins to be crystallized not only on the surface but also in the bulk at lower temperatures. Consequently, the novel transparent machinable mica glass-ceramic can be obtained by heating the glasses having the chemical composition of Ca 0.6 Mg 3 Al 1.2 Si 2.8 O 10 F 2 and K 0.01 Ca 0.595 Mg 3 Al 1.2 Si 2.8 O 10 F 2 . As a larger amount of calcium-mica is separated, the bending strength decreases and the fracture toughness increases. Furthermore, by replacing K + ion instead of Ca 2+ ion in the interlayer of calcium-mica, the interlayer bonding strength becomes high, resulting in the increase of the bending strength.