Unidentified Rock

[Huggerwoggle]

Hello Everyone, any help would’ve appreciated. I purchased a small rock collection from a thrift store. There were some petrified wood, quartz, Arrowheads but what caught my eye was this rock which has some weight to it. It measures around 4 inches long and 2 inches thick. It appears to have remnants of coral embedded on it. It is not magnetic. It is much heavier than lava rock. Any help would be appreciated.

 

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[Moesia]

Can a knife/nail scratch it?
It's true surface is obscured by weathering, you would have to break a peace of it to have a better look. It looks like it is mostly made out of quartz.
Try to scratch that milky white mass with a knife/nail.