Density of rocks seems to top out around 3.2 g/cm³[0], but if you could find a way of scanning for material that was 99%+ Carbon and which had the density of diamond (3.51 g/cm³) and you could detect very small buried items, then that might be a start.

https://chem.libretexts.org/Ancillary_Materials/Exemplars_an...

> surely theres a threshold density for diamonds that is far above rocks

Nope, diamond is 3.51g/cm³, some rocks are in the same density range.

Diamond's density is 3.5 g/cm3, nothing special.