Glowstone

Glowstone is a light emitting block that emits a light level of 15. It is naturally generated in the Nether and can be found hanging from ceilings and overhangs. It can be used for interior lighting as well as decoration.

A glowstone block on the ground (via screenshot)

How to get glowstone?

Glowstone is naturally generated exclusively in the Nether. It gets generated in crystalline blobs on the underside of ceilings and overhangs. The only way to pick up glowstone blocks is by breaking them with a tool that is enchanted with silk touch. If broken with a non-enchanted tool, the block will shatter and drop glowstone dust instead.

A cluster of glowstone blocks hanging from the ceiling in the nether (via screenshots)

You can craft glowstone blocks by using glowstone dust. To do so, simply place 4 of them in a 2x2 formation in the crafting grid. Another source of glowstone blocks is from clerics that have been leveled up to the journeyman status. They will sell you one glowstone for four emeralds. Wandering traders have a chance of selling glowstone blocks as well, for two emeralds.

Crafting glowstone (via screenshot)

What is the use of glowstone?

Glowstone blocks are luminous blocks that provide light and can be used in some crafting recipes. They are mostly used as light sources since they emit a light level of 15, which is the highest light level in the game. Glowstone is a necessary component for crafting redstone lanterns and respawn anchors.

Using a glowstone to make a redstone lamp (via screenshot)

Conclusion

Glowstone is a block that can be used for a lot of things such as decoration, lighting, and crafting a few other items. Since the nether is quite hostile, getting it from there can prove to be challenging, so the recommended source would be trading with villagers.