Definitely I'll bring an expert to confirm, but I'm pretty confident is Terrazzo Stone. Terrazzo was incredibly popular in my country during the 50's and 60's to the point almost every house built during those decades had Terrazzo floors. Right now I'm at my Mother in Law apartment walking over a terrazzo floor.

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That said, the aggregate on the area I uncovered seems to be way smaller of what is commonly see on my country so I'll confirm with an Expert. Thanks