[Gwathagor]

Well...one Silmaril did fall into the earth. Perhaps that was nearer to Erebor. Or was that one consumed by magma? But, even if it wasn't, I think it's a good point that there would not have been enough time for more crystal-junk to accumulate around the gem, which is necessary for the transformation into the Arkenstone. I rather doubt that anyone other than Feanor would have had the ability to alter a Silmaril, so without extra crystal formation around it, the dwarves probably wouldn't have been able to do any cutting or shaping - nor would they have wanted to.