rock has dark blue color inside

[smilodon]

Anyone know why some rock has dark blue color inside? I used some like that before but this time I want to figure out what exactly causes this.

 

[G Santana]

Anyone know why some rock has dark blue color inside? I used some like that before but this time I want to figure out what exactly causes this.

I have similar rock that's blue all the way through. It's the skeleton of an old coral that just happens to be blue.
I got that coral some 30 yrs ago, I don't see much for sale these days.

 

[G Santana]

Blue ridge coral, just remembered the name!!!