Castle Diorite is an item added by HermitQuest. It is used to summon the Hermit Zueljin.
Castle Diorite appears only near hilltops in Extreme Hills biomes, and always appears as a vertical stack of 2 or 3 blocks. It looks just like vanilla diorite.
The pillar of Castle Diorite can often spawn with Cosmic Dust on top of it.