The crystaldune gemling’s trait Oscillation has no impact on spell gem potency. The trait reads “Your creatures’ Spell Gems are 70% more potent when they have an odd number of Charges, and 70% less potent when they have an even number of Charges. This trait does not stack.” However, this doesn’t change the damage or stat increases/decreases of spells you cast, regardless if it’s this creature or another creature and whether the spell gems are ethereal or not.
You can test this by going into a realm with a crystaldune gemling and casting some spells. Even as charges decrease, the numbers in the battle log do not change. Furthermore, these numbers are the same as they would be without the oscillation trait (neither the 70% boost for odd charges nor the 70% penalty for even charges takes effect).
This is on version 1.0.5 on Windows.