When you place resonating ore in the smelter it will produce RCL. This liquid is the crucial ingredient for growing your own crystals. The quality of the final crystal will depend on the attributes of this liquid:
Only liquids with 100% quality can be grown into the best crystals. Quality is determined in the smelter. Only with the right amount of lava (tank about half full) will you produce optimal quality RCL. Putting too much lava in the tank will make the smelting process too hot which introduces impurities in the liquid.
Note that there is no way to increase quality later so it is very important that you get quality right from the first step.
The purity of the liquid determines how much radiation and pollution you will get when generating power from a crystal. It is important to try to get the purity as high as possible. In addition, crystals with higher purity will generate more power at a higher RF/t as well.
With the right amount of lava the smelter will produce liquid with only 10% purity so you need to use the purifier to improve purity.
The strength of the liquid determines how much power in total this crystal will be able to generate. The smelter will always produce liquid with strength 10%.
The efficiency of the liquid determines how efficient the generated crystal will be able to produce its power. In other words, the higher the efficiency, the better the RF/tick. The smelter will always produce liquid with efficiency 10%.