Here's a quick test of the range of each pigment. There is a noticeable difference is hardness among these pastels, and especially noticeable towards the pure pigment end of each set. The yellow ochre and viridian were particularly scratchy. The black iron oxide was very soft, buttery is probably a good analogy, with the sticks breaking up easily. The white was soft and I think that influenced the light side of all the pigments.