Any ideas what this big rock may be?

[Sandytoes]

I have had this rock for over 50 years and would love to know what it is. I found it as a child on the shore of Sand Lake near Max, Minnesota. That's way north. The rock is slippery smooth. Sometimes it looks black and sometimes dark navy blue
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[hvacker]

Looks like a pudding stone.

 

[Oakstrails]

My guess:
Snowflake Obsession

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[smokeythecat]

Welcome. Puddingstone may be right.

 

[Eu_citzen]

Porphyry.

 

[Sandytoes]

Thank you all for your great suggestions.