Macor is a composite material made of fluorophlogopite (a type of Mica) in a borosilicate glass matrix (such as used in test tubes and Pyrex) in a ratio of 45/55 respectively.  The randomized microcrystalline structure allows tools to excavate micron-sized portions without cracking and fracture, leading to very exacting tolerances. Macor is composed of: 46% silica (SiO2) 17% magnesium oxide (MgO) 16% aluminium oxide (Al2O3) 10% potassium (K2O) 7% boron (B2O3) 4% fluorine (F)