I'm Native, did a lot of N.A. Markets, have never seen this or
heard of it. I'm curious as to what it looks like.
The book, "Navajo Jewelry, a Legacy of Silver and Stone" by Lois
Essary Jacka and Jerry Jacka has an unassembled shadowbox belt buckle
by Harvey Begay on page 62. I know you have to find a copy of the
book to see that but it shows how to make shadowbox jewelry.
The best I could come up with quickly is the following. Shadowbox is
something like overlay, except with overlay the two layers are
surface-to-surface flush. With shadowbox, the top layer floats over
the bottom layer rather than rests on it. Take a look at the "Corn
with Coral" pendant, top right here:
"Corn with Coral" is overlay. Picture the top silver layer more
domed and the blackened bottom layer flatter, so there would be empty
space between the two layers (except at the circumference where they
are joined). That would be a shadowbox effect. The Harvey Begay piece
is more complex in its construction, but that is the general idea.