Origin of the Gemstone Dragons
by Simone NeriGemstone Dragons originate from a cabal of ambitious dragons who, in the Blackmoor era, manage through powerful magics to improve themselves, becoming more powerful in arcane magic and supernatural powers. In order to dominate Mystara, they trigger a war between the Blackmoor Empire and other dragons, during which the First Dragonlord sees his rise. Then (but not before the dragons are nearly exterminated) the Gemstones treachery is discovered, and the dragons join forces with the Blackmoorians against them. The Gemstones are defeated and exiled in the outer plane of Veydra.
An unknown number of centuries later, the being known as the Overlord conquers Veydra and mentally enslaves the Gemstone Dragons, making them the generals of his army, which invades Mystara at the beginning of the 6th century AC (according to LoZompatore's calculations, the first novel should take place in AC 504, and the third should end in AC 513). The Overlord's army is defeated and himself destroyed thanks to Thelvyn/Thelvaenir/Diamond, and the few Gemstones surviving the war are freed from the Overlord's control and allowed to live on Mystara.
I wrote a long history of dragonkind the last year, which is nearly complete, and which tries to collate the different version of dragonkind origin and history. I'll have to publish it one day or another...
My opinion, anyway, is that there shouldn't be Gemstone Dragons on Mystara between BC 3200 (the approximate date of their exile) and AC 513.