In general, most natural black lava stones will still have some modifications. The most common modification is the addition of wax on the surface of the lava stone. Wax is added to improve the appearance of the lava bead and make the stone less "rough". However, there is a lot of back-and-forth regarding the effects of this wax on the absorption of essential oils.
If you lava stone is an beautiful black color with a shiny and relatively smooth surface, then they are likely coated in a thin layer of wax. When wax is removed, lava is typically a dull gray color and less aesthetically appealing. Most of our lava stones are covered in a thin layer of wax and we have had no problem with absorption when essential oils are applied properly and given time (5-10 minutes) to absorb the oil.
Waxes and oils are both part of a larger group called lipids (or fats). When wax is added to lava stones, it creates a lipid barrier around the stone. While water and water-based materials are unable to bypass this layer, other lipids are "lipid-soluble" and able to move through (or permeate) this wax layer. Therefore, since essential oils are lipid-soluble, they can move through this wax layer and absorb into the lava stone.
As a side note, the membrane covering your cells are also lipid based and therefore, essential oils are able to permeate skin cells. Therefore, when adding essential oils to lava stones for scent purposes, we advice customers to add 1-2 drops to the lava stones while the stones (or bracelet) are sitting on a paper towel. This will allow your paper towel to absorb any excess oil (and NOT your skin).
Let us know you experience with waxed lava stones in the comments.