Igneous and metamorphic rocks that contain no minerals but have particularly high Rb:Sr ratios (see rubidium–strontium dating), or rocks that are too fine-grained for mineral separation, can be analysed as whole-rock samples. Whole-rock samples from different parts of the same body generally differ in rubidium content and the ⁸⁷Sr:⁸⁶Sr ratio of each can be plotted as a function of its ⁸⁷Rb:⁸⁶Sr ratio in an isochron diagram. At the time of the initial crystallization different parts of the sample, regardless of rubidium concentration, would have had the same ⁸⁷Sr:⁸⁶Sr ratio and hence plot as a horizontal line. With the passage of time ⁸⁷Rb would be lost and corresponding amounts of radiogenic ⁸⁷Sr gained. As the ⁸⁷Sr:⁸⁶Sr ratio changes in each part of the rock, the slope of the isochron increases progressively, providing a measure of the age of the crystallization. The intercept of the isochron at the ordinate indicates the isotopic composition of common strontium at the beginning of the process.