HoMM3 HD is a mod (add-on) for the game Heroes of Might and Magic 3.

Who doubts that gems are given by the Naga bank in vanilla HoMM3? Open the editor, go to the section with all objects (where dwellings are listed at the beginning), find the Naga bank, right-click on it, and enjoy the description about gems. If anything, this information has been around since RoE, and possibly even since the beta version of HoMM3, because, for example, you can see an early overpowered version of 100 guard creatures and 5 angel rewards (and other fun data in other descriptions). An obvious game design choice is to give resources in banks that are related to the 7th-level creature of the same alignment as the bank. If anyone remembers, the Human faction in RoE (and the Undead faction similarly) had 5,000 creatures without any resource cost – their bank didn't give any resources. The Barbarian and Swamp factions, as neutral alignments (i.e., with their own specific features), had a different mix of rewards, even though resources were needed for the 7th-level creature (but they repeated existing ones, so their banks were also specific), and, unlike other factions, the Barbarian and Swamp factions cost 0 resources without an upgrade and 1 with an upgrade (other factions, except for Humans, cost 1/2 resources). The Undead bank is a whole other story (but again, their resource was already used, and the creatures were weaker and cost 0/1 resources). Therefore, all these things are dictated by a certain connection and symmetry in the balancing of the cities, and sulfur in the Naga bank is complete nonsense in this context, which is why people say it's a bug. For English-speakers. To find whether gems are primal for the Naga bank or sulfur, open h3maped, find the Naga bank object (the section where dwellings are), and right-click it – you'll see gems for the Nagas, and other RoE-stuff in many descriptions (like 100 griffins in the Citadel and 5 angels as a reward). So, resources in banks have a specific place in the game designer's idea, which tends to balance all the possibilities among different alignments (factions). And sulfur is a bug.