[Formendacil]

In the case of the Silmaril in the earth, there is good reason to have left it alone. This Silmaril was, as noted, cast there by Maedhros, presumably during the tumults that destroyed Beleriand. Aule would naturally be loath to retrieve it for the simple reason of doing more damage. He might have the POWER to go digging around in the earth to retrieve the Silmaril, but doing so would cause tumultuous damage. It's like when the Valar were fighting Melkor in an earlier time; they were afraid to fight to the fullest of their ability for fear that the resulting chaos would endanger the Children of Eru.

Furthermore, what POINT is there to the Valar retrieving the Silmarils? It's all very well to have them in your posession, but the only reason the Valar wanted them in the first place was to rekindle the Two Trees- a rekindling that would involve the breaking of the Silmarils, a task that only Feanor could accomplish.

And as Feanor is locked up in Mandos until the end of time, and since he pretty much made clear that he wasn't unlocking them in the near future, I really doubt that the Valar had any reason to go Silmaril-hunting. The Silmarils were just as safe under 1000 feet of stone and earth, or under the accumulating silt of the seas as they would be in the hands of Manwe or lighting up the sky with Earendil.