We present an original method for measuring the intrinsic dissolution rate of gypsum. We use a simple microfluidic setup, with a gypsum block inserted between two polycarbonate plates, which is dissolved by water. By changing the flow rate and the distance between the plates, we can scan a wide range of Péclet and Damköhler numbers, characterizing the relative magnitude of advection, diffusion and reaction in the system. We find the dissolution to be unstable, with a formation of a characteristic fingering pattern. The dissolution rate can then be calculated from the initial wavelength of this pattern. Alternatively, it can also be estimated from the time it takes for the gypsum chip to get completely dissolved near the inlet channel. The method presented here is general and can be used to assess the dissolution rates of other minerals.