Article at Dick Blick is called “Glass Fossils”
Here is a link to the “Glass Fossils” instructions at Dick Blick:
However, please note that this process DOES burn away the leaf or flower,
it just leaves a carbon footprint of it. I would not know how to embed a real
leaf or flower, because at the temperature of melting glass, everything with
living tissue is going to burn. I have, however, seen people who have made
flowers with a torch (and a heavy dose of skill) and then embedded those
glass items into a large block of clear glass. Maybe that is what you have seen?