Amethystle

Amethystle is a plant found in Diamond Disaster. It explodes in a 3x3 area, covering both plants and zombies in amethyst, boosting their HP (boosting them by x4 health if a normal plant, x2 health if a defensive plant and x2 for zombies) but making them slower, the amethyst will go away after either being destroyed or after a minute and 30 seconds.

Strategies
It's a good idea to use these by defensive plants so their HP is boosted while it could also be used on zombies to slow them down, at the cost of them gaining more HP in the process. It can also be used by attacking plants however, the plant will become slower. While it doesn't "boost" the health of plants as much, it's best to use the Toughberry on these types of plants.

Despite being unlocked in Diamond Disaster, if a Miner Zombie is seen there, it's best not to use it due to Miner Zombies being able to automatically destroy Amethyst covering both plants and zombies with his pickaxe. Get rid of these zombies before using an Amethystle if you really plan on using it.

Triva

 * The plant was "given" by ScribbleMasterer.
 * The entry mentions Drill Pickle, another plant from the world it's in.