Perlite.

Volcanic glass expanded 4–20 times by heating to about 850–900 °C, producing light, porous white granules used to aerate growing media.

What it means for your plants

Perlite is pH-neutral and chemically inert, so it adds air space but no nutrients; 20–50% by volume suits most houseplant mixes. It floats to the surface over time, and dry perlite releases fine silica dust, so wet it before handling.

Put it into practice

Perlite is light, cheap and floats; pumice is heavier, holds some water in its pores and never breaks down. Compare them for aroids, succulents and large pots.