Ice crystals or algae?

[The QCC]

If you guessed algae, you are correct.
The specimen is a piece of fossil algae viewed on edge with a top light and a horizontal spot light.


The HTML version.

[Pau]

Ice crystals or algae?

BTW not ice but crystals for sure, likely of calcite. Most fossils are made of mineral crystals despite preserving more or less aspects of the original organism morphology.
In your specimen the only remain of the original structure I can guess is the banding in the front side shown in your last picture, maybe a stromatolite, but not sure.
Are you sure that it's an algae fossil and not just a calcareous banded formation typical of karstic cavities?

[The QCC]

The algae fossil was purchased from the Rock Shed.
This is the description that came with the specimen.

[carlos.uruguay]

Absolutely interesting