That is a type of clay which is super-fired to create a porous texture. It's heavy enough to provide secure support for your plant's, but still light weight. Grow rocks are a non-degradable, sterile growing medium that holds moisture, has a neutral pH, and also will wick up nutrient solution to the root systems of your plants.Extremeley stable in both pH and EC

• Made from 100% natural clay
• Strong structural integrity means that they are less likely to break and compact, which helps to prevent dripper systems from getting blocked
• Pre-washed to aid in stability
• Drain freely and do not hold any excess water, providing good oxygen levels around the root; suitable for flood and drain systems, multi-pot and drip feed systems