Diffuse-reflectance spectra have been measured for a series of samples of Carrara marble experimentally deformed under different cylindrical stress ( P = 0, 100, 250, 500, 980 bars). The creation of point defects that results has been shown up classically by irradiation with β rays (40 krads), thus producing a typical blue coloration linked with the formation of colour centres. The diffuse-reflectance spectra, measured on powders with a microscope-spectrometer in the visible range (400–800 nm), allow the determination of the absorption spectra by means of the Kubelka-Munk function. These absorption spectra have been measured for each of the deformed samples, as well as for different fractions of a very deformed specimen subsequently heated at temperatures between 100 and 500° C for a fixed time. In the same way, tectonised crystalline limestones, of various origins, were studied without any other treatment than the irradiation with β rays. From this study the following preliminary conclusions have been drawn: 1. (1) The absorption spectrum of an undeformed but merely irradiated specimen of crystalline limestone is practically monotonous, but in the deformed specimens a broad band of absorption appears, having a maximum at 620 nm with several shoulders, the chief of which is at 520 nm. 2. (2) This absorption band shows the existence of colour centres, the density of which can be estimated relatively by means of the chromaticity coordinates x and y of the C.I.E. obtained from the diffuse-reflectance spectra (C.I.E. = Commission Internationale de l'Éclairage). 3. (3) An overgrinding of calcite generates defects that have the same spectra as those produced during the experimental deformation. Consequently, in obtaining the powders of grain size 50–80 μm needed for the diffuse spectrometry, great care must be exercised. 4. (4) For a given confining pressure, the defect density is proportional to the deformation rate. 5. (5) One can calibrate the effect of the annealing of crystalline limestone deformed at 1000 bars by means of the ratio Q of the intensities of the absorption peaks at 510 and 620 nm. The curve Q = ƒ(t°C) is exponential. The experimental curves show the density of the point defects created by the deformation on the one hand and on the other their healing as a function of the temperature. Further studies are being carried out in order to determine how far the plotting (on these experimental calibrations) of the chromaticity coordinates x and y and of the ratio Q of natural tectonised crystalline limestones allows the estimation of the latest orogenic pressures to which they have been subjected, as well as the temperatures reached since the last tectonic phase.