Materials
There are seven materials in the game, six of which are currently obtainable. In the inventory screen from left to right the six obtainable materials are: 
Gold, 
Wood, 
Stone, 
Iron, 
Crystals, and 
Dust. Materials have a variety of uses, all of which are related to the different buildings and shops in town.
How to Obtain Materials
Gold can be acquired from valuables, breakables, resource caches, equipment chests, and found loose on the ground.
Wood, Stone, and Iron can all be acquired from resource caches and found loose on the ground. Some Wood can be acquired by breaking furniture in later levels.
Crystals and Dust are acquired by having Magic Shop tier 1 built. At the end of a run, common/uncommon trinkets are converted to Crystals and rare/epic trinkets are converted to Dust.