Hi, Anita,
Sorry for my late response; I must have missed the email alert I receive when a Natural History Item in uploaded to our Mearto web page.

From images, I believe your example is a section of slag glass. An extrusion of molten glass silica from an industrial production. I believe it is slag glass due to the chipping patterns and bubbles found in the example. A natural crystallization would chip away in a different pattern, and while pockets of air and absence can exist in natural formations, it is fractured crystallization breaks, different from an air bubble.

Slag glass can be found in former iron-producing areas, such as near old foundries and smelting sites. Often it is found along the shores of North America's Great Lakes, as these were often industrial areas.

This example measures 10" X 8" X 3.75", no weight is given. A section of slag glass this size can be collected for display; in an aquarium or desk ornamentation setting. A possible increase to the FMV can be achieved from including the excavation source of the example. Slag glass is collectable for its unique appearance as well as it's association with a region or manufacturer.