I will be quick and concise. From the moment Fizban's Treasury of Dragons came out people were excited about the new draconic lore, but it also raised questions about where that third category of true dragons fit into the grand scheme of DnD worlds... And Exandria is no exception.
On the occasions in which Matt has been asked about it, he has not commented on it, but in the 3rd episode of ExU Calamity we have something very interesting: Nydas invokes the spirit of Sha'korzhan, the dragon that lived in what is now Avalir, and the spell he uses grants him resistance to necrotic damage. The spell in question is Summon Draconic Spirit, and it allows you to summon a dragon spirit... Chromatic, metallic, or gem. And only topaz dragons, a type of gem dragon, have damage and resistance associated with necrotic damage.
Does this mean that the gem dragons are finally confirmed as part of the world of Exandria? Will they still exist or did they go extinct before the Calamity? What is your opinion?
EDITED: New information: In a second summoning Sha'korzhan has a psychic breath weapon, which would identify it with an emerald dragon... So the thing is, maybe Nydas summoned his spirit and Lou (as the player) chose one of the three kind of dragons instead of a specific type, using the abilities of gem dragons in general. What do you think?