EDIT: PHYSICS BREAK: Monster can go onto the upstairs in the map that holds the Gemstone Cavern Secret Life Gem.
EDIT 4: I was talking about the Metal Chest. Went down to the bottom and found the Wooden Chest too. Oh, it's not a Life Gem, and I can't raft in these waters. huh.
I wasn't sure which one was which, and I thought you misspoke, yeah. I went looking in my Journal, and knew that the Road wasn't the right spot. To find the name, I had to look at the Gem Bat bestiary entry.
EDIT: Strange Material Island chest can be opened by Raft, so I got confused by Bridge talk.
EDIT 2: Found the Metal Chest in the Gemstone Cavern. Now I can make full use of my Rings of Reprisal at base HP!