Solid Stone

Solid stone can be found while mining and is used in Blacksmithing and Engineering and Jewelcrafting.

Source
Found in mithril veins.

Solid stone as an Ingredient
Solid Stone is used in the following recipes:

Blacksmithing

 * Solid Grinding Stone
 * Solid Sharpening Stone
 * Solid Weightstone

Engineering

 * Solid Blasting Powder

Jewelcrafting

 * Solid Stone Statue

Other
You need one solid stone to create the Elune Stone using Elune's Lantern from the season quest.