Why is this coral fossil colored like this?

[moodorf]

I know that what determines fossil color is the chemical composition of the rock, but why the many colors in this coral fossil I found today? It's especially noticeable on the top and bottom of the fossil. Anyway, I'm curious. 

 

 

EDIT: Oh, and upon further cleaning I noticed it sparkles.... the mystery deepens haha

Edited August 14, 2021 by moodorf

[Tidgy's Dad]

The original structure has been gradually replaced by calcite and the interiors of each corallite also replaced and infilled with this mineral. Calcite sparkles. 

This did not happen all at exactly the same time, it would have been a slow process over a considerable period of time. Impurities in the percolating water, such as traces of iron, would have become trapped in the forming calcite and, depending on their composition and quantity, caused differences in colour.  

Edited August 14, 2021 by Tidgy's Dad