The Malaita shell on pebbles, gravel, and the lava flow on which the house sits. From a recent geological map, I've learned that the lava flow underlying this house (and the neighborhood) is the "Punahoa" flow, which is about 4,000 years old. I like knowing that.
As an aside, I'll note that I'm in a period where the HDI doesn't interest me as much as it usually does. I'm aware of having produced some rather ordinary shots lately. I don't want to break the streak, so I'll soldier on until my visual sense is rekindled.