Aloha,
In the 1980s, a lava flow covered a remote mountain road that provided service access for power lines that ran to the island's correctional facility. As the utility poles burned and fell, their ceramic insulators fell to the ground and broke. You can still find a bunch of them around. The first time I spotted a pile of shards while hiking, I thought someone had brought a crockpot 4 miles from the nearest road and dropped it! After I found a few more piles I realized what they were.
Here's a couple pictures of a big piece I picked up. Anybody know if they would make good knapping material? I don't really know what I'm doing, I've tried to get a few flakes but it is really hard stuff. If it is good I thought I might find this chunk a new home with someone who knows what they are doing.