The present paper introduces a method of quantitative visual colour analyses for the alkaline feldspar, nepheline and sodalite of ornamental rocks, using digitalised images of polished surface obtained by means of direct capture method. Initially, each pixel colour of the selected mineral has been analysed in RGB (red, green, blue) format by means of graphic softwares. The results show the colour dispersion intolerably wide for one determined mineral, indicating that direct application of GIS system based on the RGB is improper for this purpose.

The detailed study of the visual colour property of the minerals reveals that, in spite of the wide variation in the RGB values, the R/G, G/B and B/R ratios are relatively constant. On the RGB triangular diagram, the colour data of the minerals fall in a small area.

In the HSB colour system, H and S parameters show only a small dispersion, in which, the H is the most stable. In this system, most of the colour variation is attributed to B (brightness) parameter. In this sense, the H (hue) is the best and the S (saturation) is the second best parameter to represent quantitatively the colour. Consequently, the HS diagram is the most recommended method to evaluate the visual colour of the minerals constituent of ornamental rocks.

Palavra chave: rocha ornamental, cor de mineral, quantificação de cores, RGB, HSB