Terrarian Might

A screenshot of a naturally generated Life Crystal, growing in a mineshaft near a chest minecart.

A screenshot of a growing Life Crystal surrounded by red heart-shaped lanterns. Notice how the cracked ones produce less light than the undamaged one on the left, while the fully broken one on the right produces no light at all.

You can rarely find Yellow Willow Trees in Forests and Plains. If you're having trouble finding them though, you can also get them from Wandering Trader trades.