The effect of Ca²⁺ and Fe^2+/3+ substitutions for Mg²⁺ on the crystallisation of fluortremolite (Ca₂Mg₅Si₈O₂₂F₂) glass ceramic was investigated by the melting of selected glass compositions at 1550 °C and thermal treatment at 950 °C. Ca²⁺ is essential to generate the favoured crystal phase fluortremolite. The needle-like crystal morphology can be achieved by adding iron oxide and results in a fluoractinolite crystal phase. The DTA, XRD, SEM and TEM methods were used to identify the mechanism of phase separation of these high fluorine-containing compositions and understand the generation of the selected crystal phase. The substitution with chromium, indium, manganese and nickel offers a variety of properties for this fluoramphibole glass ceramic and as high-strength material for technical application.