I bought a bag of lava rock at walmart I think. It said something about landscaping rock on the sack. Some of them I had to break because they were just really huge. I guess how big they need to be depends on how big the pot is. You want them big enough that they won't get buried in the soil. I put them in plumeria pots because I needed something to keep the water from splashing on the plant when I watered but they are good cat deterrents too and I like the color.
