Mu You had actually rerolled this reward many times with the Pocket Watch. He discovered that apart from the experience book and 50 fire resistance, which were guaranteed drops, the other two rewards could change.
The purple Sky Set piece could randomly be for any equipment slot. As for the evolution gems, Mu You had initially assumed that killing the Flame Lord would guarantee Flame Gems. Only after several more rerolls did he discover otherwise: Frost Gems, Thunder Gems, and gems of other elements such as wind, earth, and wood could also appear. A gem's element would directly affect the direction of the equipment's final evolution.
Drops varying like this could only mean that these gems did not drop directly from the Flame Lord, but were "stock" the Fool ancestors had stored somewhere in advance. Each time the Flame Lord was killed, a random portion would be handed out as a reward.
That also meant the total number of evolution gems was likely limited. Once the stockpile ran dry, there would be no more rewards.
Mu You was not yet sure exactly how many evolution gems could be farmed, but this boss only respawned once a week. Even if every floor below had a boss, that would only make seven in total.
In other words, the entire Dungeon could produce at most seven evolution gems a week, for tens of millions or even over a hundred million players across the city to fight over...
By that calculation, these gems were still extremely scarce. After all, one person needed more than a dozen gems to put together a full set of orange equipment. Even if all the resources were pooled, a year or two of development would only equip a tiny handful of the very top players.
For Mu You, even