From what I've read, supposedly a diamond is possible, although I've never heard of anyone finding a natural diamond in Hawai'i. You're not just going to find one laying out on the ground, not in Hawai'i. Olivine crystals, sure. Obsidian, sure. Olivine is easy to find.

As for gold, don't think so. Pre-contact Hawaiians didn't do metalworking, I just don't think there are useful metal ores to be found in the rock there. I've read that gold can be found in a type of rock that forms beneath stratovolcanoes and which is associated with subduction zones. Hawai'i isn't by a subduction zone, and Hawai'i volcanoes aren't stratovolcanoes.