If you combine two like ores, it will always turn into the next level type.
Common + Different Common = Uncommon
Uncommon + Common = Rare
Rare + Common = Legendary
Legendary + Same Legendary = Next Level
The top section: This is your main ore that you are fusing another ore too. You should place your highest ore here.
The middle section: This is the list of ores you may combine to your main ore. You may only combine lower leveled ores to your main ore. If you fuse the ore to the first choice, it will result in a quartz. Under each choice, you have a success % rate.
The bottom section: This is what the result of your fusion will be.