Some words about material quality
Materials can be of different quality level :
Simple materials are the lowest quality materials and appear on the screen as grey. 4 simple materials can be fused to make one usual material;
Usual materials are quality level 2 materials and appear on the screen as white. 4 usual materials can be fused to make one unusual material;
Unusual materials are quality level 3 materials and appear on the screen as green. 4 unusual materials can be fused to make one rare material;
Rare materials are quality level 4 materials and appear on the screen as blue. 4 rare materials can be fused to make one epic material;
Epic materials are quality level 5 materials and appear on the screen as violet. 4 epic materials can be fused to make one legendary material;
Legendary materials are the highest quality materials and appear on the screen as yellow. They can't be fused anymore and should be used to forge the best equipment available for the hero.
Higher quality materials grant the possibility to forge equipment with stronger abilities. For example, one Elixir used on 40+ level equipment grants the following bonuses to food production : 20% (simple), 40% (usual), 60% (unusual), 80% (rare), 100% (epic), 150% (legendary).
It is noted that while the equipment bonuses grows regularly from simple to epic qualities of materials, the bonus gap to abilities can be much higher between epic and legendary materials, encouraging players to fuse materials up to the highest quality.
You can obtain materials in different ways :
Complete personal, clan or premium tasks. The quality of the material cannot be higher than the level of the completed task (for example, you can get simple, usual or unusual material by completing a unusual task).
Gather resources at a resource point. The probability to get quality materials goes higher with the amount of resources gathered and the level of the resource point. It is possible to get all qualities of material from simple to legendary.
Hit or kill invaders. Each hit on an invader generates one item which can be a standard or invader specific material. Invader kills generate a random number of items, up to 9, which can include one or more materials. Level 1 only generate simple materials, while you can get up to legendary materials at level 6 invaders. Two criteria must be met to get materials better than simple quality : the level of you hero's sustained attack (high quality materials are obtained when you reach 300% sustained attack and more) and critical chance (you get much higher quality materials with crits).
Open equipment coffers. The quality of the material that can be obtained depends on the type of coffer. Coffers can be purchased for gold, only the coffer of usual materials can be obtained at the cost of 6000 loyalty points at the Clan Store.
Gather resources at invader's lair. The probability to get quality invader's materials goes higher with the amount of resources gathered and the level of the invader's lair.