Obsidian

Obsidian is a tough block known for its long breaking times and explosion immunity. Obsidian can be used to create a nether portal.

Breaking
Obsidian can only be broken using a diamond pickaxe.

Post-generation
Obsidian is formed when water comes into contact with a lava source block.

Obsidian is created when a new nether portal is generated in the nether or overworld.

Usage
Due to the high blast resistance of obsidian, it can be used in making explosion resistant structures. It also cannot be pushed or pulled by any kind of piston.

Nether portal
Obsidian can be used to make a portal to the nether. A nether portal frame must be 5 blocks tall and 4 blocks wide with 6 blocks of air in the middle. Once the frame is built it can be opened by using a flint and steel on the bottom inside the frame.