Glass Diamond

Glass Diamond is a Mythical catch from the caves of Northern Expedition, specifically Overgrowth Caves and Frigid Cavern. The wiki describes an incredibly unique translucent diamond from the Earth's most frigid caves. No bait, weather or season is listed, so the location is the whole recipe.

It is a small item with a strong rate. A typical catch weighs 4 kg, between 2 kg and 10 kg, and it sells at C$300 per kg, giving an average of C$1,200. That ranks 306 of the 742 documented fish, so it sits in the upper-middle range, while its maximum weight ranks 470 of 663, on the light side. A catch grants 375 base XP.

At C$1,200 it is a modest earner for a Mythical, so it is not worth a trip on payout alone. Its real draw is being a gem rather than a fish, which makes it a fun collection entry for anyone who explores both caves. If you are already working through Northern Expedition's cave zones, it is a pleasant extra, and selling it yields a reasonable sum. If you are looking for XP, 375 per catch is a small reward.