It was a children's craft kit, from around the 1970s
It was plastic/resin kit - you could put eg flowers in clear plastic, then give a background (blue or yellow, don't remember any other colours) and then you had a lovely object, which you could turn into a pendant or stick on to a ring.
Can you still buy it?
Resin kits I've found are much bigger and more elaborate. Is there such a thing as a little one just for an afternoon's nostalgic messing around?
Or if anyone does this stuff seriously, any recommendations inexpensive entry-level versions?