Opal

Opal is a Gemstone-rarity catch from Ancient Isle, and it is obtainable now. The wiki lists very little about conditions: no bait, weather, season or time is given, so the guide is straightforward: go to Ancient Isle and fish. The in-game text calls it an iridescent gemstone with an array of different colors.

It is small: a maximum weight of 5.1 kg, ranking 546th of 663 fish. The value comes from its price per kg, C$156.25, which gives an average worth of C$500, 390th of the 742 documented fish. That is a middle-of-the-table payout, though not high enough to change your income.

Ancient Isle also produces Moonstone and Ruby, which share the Gemstone rarity. Gear is easy to find in the zone: Krampus's Rod, Phoenix Rod and Stone Rod are all sold there.

Verdict: a decent, low-friction catch. Because no conditions are listed, it works well for filling a quiet fishing session in Ancient Isle, and the three rods sold nearby give you options. If you are collecting gemstones, this is part of a three-piece set with Moonstone and Ruby. If you are only after money, there are catches that pay many times more, so treat Opal as a complement and not the main plan.