In order to craft a piece of equipment, you need to use certain crafting materials. Each piece of equipment requires four distinct crafting materials, which may differ in quality. Each item you craft grants you a certain probability of obtaining an item of gear of a specific quality, depending on the materials used. The greater the difference between the quality of the crafting materials you use, the lower your chances of crafting an item of the desired quality.
In the example below, you can see that using lower-quality materials reduces your chances of crafting a high-level item significantly. In this case, using these materials, you have a 75% chance of crafting an item of epic quality when using three items of a higher quality.