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
.

 

Amazon Forum Fav 👍

Treasure of Victoria Peak - Grab it through Amazon!

[hvacker]

Looks like a pudding stone.

 

[Oakstrails]

My guess:
Snowflake Obsession

Sent from my LG-H901 using Tapatalk

 

[smokeythecat]

Welcome. Puddingstone may be right.

 

[Eu_citzen]

Porphyry.

 

[Sandytoes]

Thank you all for your great suggestions.