Glad you like them!
I'm not a technical wizz with glass (just know what I like
but I don;t think it is cased in clear. It looks like a central core of clear, with the colours you see on the outside, white on the inside, and the trailing over the top of the outside colours - but the trailing is not raised - it is sort of sunken into the sand and turquoise colours.
Sorry - I cannot explain it any better than that....... you can feel the trailing quite clearly, but it is still a fraction lower than the sand and turquoise layers - If you imagine a flat wet sand beach with a rivulet of water over it thats how it looks - as if it has just sunk into the other layer a little. It is definitely layered on top of them though as you can see those colours through it as the trailing blue glass is transparent rather than opaque.
have added a close up - hope it shows better what I mean