A method of determining a more accurate value of the work index of a rock sample by measuring the energy required to break the rock sample into two or more pieces, the method comprising the steps of: subjecting the rock sample to one or more impacts until the rock sample breaks, wherein the step of subjecting the rock sample to an impact comprises mounting the rock sample on a support a distance h vertically below two pivot points (113A, 113B) from which two rigid pendulums (112A, 112B) are respectively pivoted, the rigid pendulums (112A, 112B) being mounted by respective ends thereof for rotation about respective pivot points (113A, 113B) in a common plane and each rigid pendulum (112A, 112B) carrying a respective mass (116A, 116B) at an end thereof remote from a pivot point (113A, 113B), the distance from a pivot point (113A, 113B) to the midpoint of a corresponding mass (116A, 116B) being substantially h, the method further comprising raising each mass (116A, 116B) to substantially the same vertical height above the rock sample and releasing the masses (116A, 116B); and measuring the energy absorbed by the rock sample at each impact, wherein the step of measuring the energy absorbed by the rock sample for a first impact comprises measuring the respective inclinations of the pendulums (112A, 112B) prior to release of the masses and wherein the step of measuring the energy absorbed by the rock sample for each subsequent impact comprises measuring the respective maximum inclinations of the pendulums (112A, 112B) subsequent to the previous impact of the masses (116A, 116B) with the rock sample, wherein the pendulums (112A, 112B) are momentarily at rest before swinging back down towards the sample, and the step of measuring the energy absorbed by the rock sample for a given impact comprises measuring the respective maximum inclinations of the pendulums (112A, 112B) subsequent to impact of the masses (116A, 116B) with the rock sample; characterized in that the method further comprises the step of summing said measurements of energy to obtain a measurement of the total energy absorbed by the rock sample and using the measured value of the total energy absorbed to calculate the work index of the rock sample. subjecting the rock sample to one or more impacts until the rock sample breaks, wherein the step of subjecting the rock sample to an impact comprises mounting the rock sample on a support a distance h vertically below two pivot points (113A, 113B) from which two rigid pendulums (112A, 112B) are respectively pivoted, the rigid pendulums (112A, 112B) being mounted by respective ends thereof for rotation about respective pivot points (113A, 113B) in a common plane and each rigid pendulum (112A, 112B) carrying a respective mass (116A, 116B) at an end thereof remote from a pivot point (113A, 113B), the distance from a pivot point (113A, 113B) to the midpoint of a corresponding mass (116A, 116B) being substantially h, the method further comprising raising each mass (116A, 116B) to substantially the same vertical height above the rock sample and releasing the masses (116A, 116B); and measuring the energy absorbed by the rock sample at each impact, wherein the step of measuring the energy absorbed by the rock sample for a first impact comprises measuring the respective inclinations of the pendulums (112A, 112B) prior to release of the masses and wherein the step of measuring the energy absorbed by the rock sample for each subsequent impact comprises measuring the respective maximum inclinations of the pendulums (112A, 112B) subsequent to the previous impact of the masses (116A, 116B) with the rock sample, wherein the pendulums (112A, 112B) are momentarily at rest before swinging back down towards the sample, and the step of measuring the energy absorbed by the rock sample for a given impact comprises measuring the respective maximum inclinations of the pendulums (112A, 112B) subsequent to impact of the masses (116A, 116B) with the rock sample;