The story is on Earth, though a good part of it seems to be in the skys. It starts in Saint Petersburg in Russia where Yoko and Emilia and the countess Olga retrieve the Tsar to bring it back to Scotland.
There they briefly rejoin Vic, Pol, Mieke, Rosee as well as Angela, before a letter sends Yoko and Emilia towards new adventures... which will set them in 1934 and within Emilia's family....
Source: Emilia (addditional details on the Russia stay, the letter, the machine used to go in 1934) [French]

Why in 1934, answer from Ebonite/Bonnie [French]:
"Why that year... because a little boy who would one day give me life was already fixing his eyes on the univers that surrounded him without understanding it, and he decided to send Yoko and Emilia there to relive for him what he does not remember... and to meet there again his parents by whom he settled his first memories..."

An explanation (in French) of the true properties of the amthyst is given by universscience.tv on their site.

Report (in French) on RTBF for the release of "Le maléfice de l’améthyste":

Some drawings:

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Some drawings (inked or partly inked):

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Some drawings (with colours hints):

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