Spongy green glass found on graves

Pebbles or Chippings

I know that weird green stuff you mention! And it’s not natural sea-worn glass. But if I wanted to describe it, I’d say ‘green glass pebbles’.

When I looked it up online I also found ‘chippings’.

You can even buy them on Amazon!

I can find two possibilities.

First, in some cultures like that of African-Americans in the American South, graves were often covered with various bric-a-brac. These would have included the green glass you describe, perhaps obtained from bottle fragments. Ross W. Jamieson describes them:

In North America the surface decoration of graves with ceramics and other objects is the most commonly recognized African-American material culture indicator of cemetery sites. William Faulkner, in Go Down, Moses, described a black cemetery with “shards of pottery and broken bottles and old brick and other objects insignificant to sight but actually of a profound meaning and fatal to touch, which no white man could have read” (Faulkner 1942:135; cf. Vlach 1978:139).

Second, you could be referring to terrazzo glass, which is sometimes used for headstones or grave coverings. This is glass fragments cemented together (Materialicious). The glass often looks bubble-like, especially if it's translucent. The general process is hundreds of years old, and used for anything from graves to countertops and floors.