OK, maybe not properly transparent, but an inclusion of fibre-optics makes it light transmitting.
It's a German product, and, from their website:
LUCCON translucent concrete is manufactured in large volume top layer concrete blocks with embedded webbed fiber optic cables, making the stone appear comparably massive as well as transparent. By cutting the blocks elements of variable size and thickness are obtained. Upon request LUCCON is available in special colors, dimensions or surface treatments, just ask us! .
I'd love to see what our members could make with this stuff, or see them replicate the idea with their own techniques.
Concrete lampshades? Concrete windows? Transparent concrete canoes?
Luccon website via NOTCOT