There are lots of different resources in Genshin Impact, and some are harder to find than others. Noctilucous Jade is an important, but rare, resource that you will need to have for certain weapons and character ascensions.
You can only find Noctilucous Jade in the Liyue region, and it will often appear in caves, and close to Mingyun Village. Just like other ores, you will need to smash it with your weapon and then pick up the chunks that fall. Searching around to the south of Wuwang Hill and Stone Gate should provide you with plenty of this resource.
The ore will look like a blue crystal that is used to the ground via black dirt and is quite easy to spot due to its size. If you are having great difficulty finding it in the wild, you can visit Shitou, a merchant in Liyue Harbor. He will sell Iron Chunks, White Iron Chunks, and Noctilucous Jade. The Noctilucous Jade will cost 1000 Mora each.
The merchant is certainly the easiest way to get this resource, but if you are low on Mora, or do not want to spend anyday, then you will need to search in various caves for it.