Disclosed is a method and apparatus for determining stone cells within a sample of wood pulp or paper. According to the instant invention a portion of the sample is irradiated with light at a predetermined wavelength or within a predetermined range of wavelengths. The incident light causes those areas of the sample that are occupied by stone cells to fluoresce strongly relative to the background matrix of wood pulp or paper. The strongly fluorescing stone cells are registered digitally using a detector, or are observed visually by an operator viewing the sample through an ocular device. The absolute number of stone cells per unit area or a stone cell size-distribution-plot is obtained using one of automated or manual image analysis.