I often receive questions about the potting mixes I use for my bulbs. Needless to say, I vary the mixes according to the bulbs I am growing, with desert species being planted in a very dry mix that is largely composed of horticultural pumice, sometimes with no organic material at all, while bulbs that grow in forest regions will require more organic material. I used to use a lot of perlite, a substance that is a volcanic material that has been heat treated to make it expand. It is very cheap, light and readily available, and I used it to lighten the mix for the bulbs, not just to reduce the weight, a consideration when you are mixing tons of potting soil and moving hundreds of pots, but mostly to allow better water penetration and air to the roots.
From some of the questions I have received I think there is a little confusion about pumice. Horticultural pumice is light grey and is sold in bags or bulk. A similar material is sold to use in horse stalls, and I believe it is called Dry Stall. The horticultural pumice I buy is very uniform with a particle size that is about 2-5mm in diameter and it has no dust or fines in it at all. The material for horses has a lot of fine material in it, and I don't think it is as suitable for growing bulbs. The whole point of the pumice is to allow more air to the roots and faster drying of the potting mix, an essential factor when you are growing bulbs from dry regions.
Lava rock that is sold for landscaping and is used in paths and as a mulch is not at all suitable for growing bulbs. It is high in minerals, some of which can be toxic to plants. It is black or red.
Perlite is a useful ingredient for potting mixes, but I am phasing it out. I have become very sensitive to the dust, and even using a mask I get a lot of respiratory problems and eye irritation from it. It breaks down over time and floats to the top of the potting mix, which pumice does not. It is far less expensive than pumice (about one fourth of the cost), but if I get better survival and less fungal rots with pumice it is well worth the price.