Amethyst Squirrel

Cavern depth, in range of housed NPCs, with better odds while it rains. In practice that means an underground town does the work for you: settle a few NPCs down there and check on them during storms. Amethyst sits at the bottom of the gem rarity order, so it shows up long before you ever see the diamond one.

Squirrels are on the hostile critter list, which the bunnies are not. Push your luck into the negative, usually by killing a ladybug, and the harmless cavern squirrel turns around and attacks you. It cannot do much damage, but it is a clean tell that your luck has gone the wrong way and you should go fix it.

Any Bug Net takes it in one swing. It has so little health that the real risk is something else killing it first, or you clipping it with a stray shot while clearing the room. Cage it and it becomes permanent decor, recoverable only by putting the Amethyst Squirrel Cage through Shimmer. It is also an ingredient for Rock Candy.