Assuming your Lumax piece was French, I Googled "lumax verre" and immediately found an auction for a similar piece, though with different proportions:
GRAND VIDE POCHE BRIQUE DE VERRE LUMAX LE CORBUSIER n°1The description reads: "Grand vide poche carré en brique de verre signé LUMAX design par Le Corbusier, rare grand modèle, bon état générale, peu de trace d'usage, un petit choc au dessus (ph.5). Belle pièce design à saisir pour collectionneur et amateur. Hauteur 10 cm ; cotés 14 cm. Un autre LUMAX plus petit est en vente sur le site Consultez mes autres objets!"
Claimed to be designed by Le Corbusier, this might well just be excessive salesmanship. Prism tiles in the US are sold, 99% of the time, as designed by Frank Lloyd Wright, though they generally are not.
This same dealer has another one similar to Margi's. They want 30 euros for the big one, which is too much for me at this time (when considing postage also).
There are also three different ones for sale on
leboncoin.fr. All three are being sold as ashtrays, though none of them have the usual grooves where coffin nails might rest. Prices are from 7 to 20 euros.
To muddy the water even further, one of them has an ugly painted band around it with lettering AIR INTER, as if it were a desk accessory; this may just be a very similar item, which wouldn't be unusual since it's a plainish design, basically just a deep hollow square dish.
Another of them has the description "ENDRIER VIDE POCHE LUMAX LECORBUSIER PERRIAND. Utilisé par LE CORBUSIER/PERRIAND dans leurs décorations dans les années 50. Inscription LUMAX dans le verre (épais) - Verre épais (1 cm) et transparent -14 x 14 cm sur 8 cm de haut. Vendu en l’état – des eraflures et des éclats", so they claim it's a 1950s era desk piece.
One of these three, the one marked Lumax and shown from the side, looks very pavement-lightish to me.
The water is fairly muddy now. Ash tray? Deck accessory?? Or pavement light???
I would rule out
cendrier since they're missing the functional grooves. The AIR INTER one seems like a bad desk accessory. The Lumax marked ones seem like pavement lights. If the Lumax ones
do have thick "bottoms", much thicker than needed to perform a mere desk accessory job, then I'd say pavement light. If they have thin "bottoms" then they would be unsuitable for use as pavement lights.
--ian