Thorium Tube

A Thorium Tube is a component created and mostly used by engineers.

Source
This item is crafted with Engineering (275); taught once by Schematic: Thorium Tube, now by trainers.

Engineering

 * Biznicks 247x128 Accurascope
 * Core Marksman Rifle
 * Dark Iron Rifle
 * Flawless Arcanite Rifle
 * Major Recombobulator
 * Masterwork Target Dummy
 * Adamantite Scope (as of patch 2.3 it doesn't require a thorium tube to craft)

As a quest objective

 * Craftsman's Writ - Thorium Tube (quest)