In out recent publication in Journal of Applied Physics, we report on an open microcavity containing a diamond micromembrane. Despite operation in the so-called diamond confined regime, where the electric field profile inside the cavity is maximized across the diamond-air interface, we demonstrate quality factors exceeding 120 000. We next develop a qualitative model describing the losses in our cavity and determine that the dominant source of losses to be surface waviness – surface textures with dimensions comparable to the cavity beam waist. The large quality factor combined with a small mode volume of 3.9λ₀³, yield a predicted Purcell factor of 170, a significant improvement over the state-of-the-art.