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In my childhood, there was a spongy (ie full of bubbles) glass of a pale-green shade which was placed covering the stone-bounded graves in more than one of the church grounds near me, is there a name for this? It's a semi-transparent (the bubbles make it so) chip of glassy substance, but the sharp edges are not so sharp as to cut skin, they seem to have undergone some sort of foaming process to make them less dense than glass and have visible surface pits that appear to be part-spherical - as of bubbles of gas that existed before the glass set.

The graves had their usual scattering of pale-green ".......", the older ones with the darker green of weeds popping
through....

The comments have lead to a perfect suggestion of an image by @KannE.

^{-Attribution indeterminate: Possibly Leo Reynolds 2019}

A generic term, be-it colloquial or slang rather than a brand name is preferred.

Most recent edits:

The chips were loose as-such, not cemented together, as I now gather terrazzo is.

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.

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!