Strange Rock Heart

 is used in crafting the Rocket Engine. You only need a total of three. It is found in the center of the Organic Rock, which is in the top right up of the world.

Strategy
To obtain the Strange Rock Heart you need a Miniaturizor MK V. It is also recommended to use a Health Regeneration Potion because you can be damaged by the Strange Rock if it grows back around you. Can use Composite Metal Support, to keep the Strange Rock from regrowing right away.

In-Game Description
That is definitely a very strange material.

Trivia
If you manage to break all Strange Rock Heart's before they regrow back the island will fall apart starting from the lower half up. So best to be careful with Explosives or after you've won the Ultimate Grenade Launcher.

Here is an interesting forum topic about Mr. Rock: https://steamcommunity.com/app/315460/discussions/0/1696046342870768826/