Amber is a mineral introduced in Thaumcraft 3. It is collected by breaking Amber Bearing Stone. The primary use of Amber is as a researching ingredient but it can also be used in trading with NPC villagers and it can be made into an Amber Block by crafting 4 Amber in a crafting grid (and 4 Amber Blocks to craft 1 Amber Brick).
Mining an Amber block requires an iron pickaxe or better, and drops one Amber.
Amber will have more uses in the future, as stated by Azanor, the creator of Thaumcraft.
Here's a video about Amber: