Besides being aesthetic, top dressing plays an important role in changing the behavior of the growing medium. It keeps the growing medium moist longer and distributed more evenly. It also keeps the temperature of the growing medium more balanced and the roots of the plants near the surface cooler. When growing Haworthias in a dry climate, topping a well-drained growing medium often results in better growth.
The particle size of top dressing should generally be larger than the one of growing medium so that the top dressing won’t move downwards and mix with the growing medium. It should not have hard and sharp edges, to not cut the bottom leaves when the plant is elongating.
Rhyolite is a volcanic rock with a very high silica content. Eruptions of granitic magma can produce rhyolite, pumice, obsidian, or tuff. These rocks have similar compositions but different cooling conditions. Because of its physical property, crushed rhyolite is often flatter than other rocks. When using as a top dressing, it forms a tighter barrier and keeps moisture in a little better than more rounded rocks (for example, decomposed granite).
Due to the mining and crushing process, rhyolite often contains excessive undesired fines. Before using as the growing medium, it should be rinsed until the runoff becomes clear.