These rocks can be used for decoration and to retain moisture for the soil, also can be placed at the bottom of the pot to collect excess water for pots with no drainage hole.
Lava rock is formed from volcanic eruptions. The porous air filled rock has a high water holding capacity as the moisture sticks to the extra high surface area. There are many usage for these amazing rocks:
1- Place them on top of the plant soil to save the moisture and for decoration as well.
2- For pots with no drainage hole: Place them at the bottom of the pot to collect excess water away from the roots because the excess water will cause root rot. When the rocks absorb the water, it will slowly release the excess water to soil.
3- You can also use them for aquariums and terrariums decoration.
The top rocks layer should be minimal to keep good air circulation for the indoor soil.