Fossil coral normally results from petrification--which is (often or usually) the replacement of the coral constituents with quartz (agate). The soft tissues were probably replaced first, followed eventually by the calcite that forms the hard mineral parts of corals (of the stony groups). This is not "hardening"--but rather is transformation, to a material (that in fact) is harder. This transformation probably takes a very long time, and happened millions of years ago.

Under certain circumstances, fossil coral can be red or reddish. This is not a result of maintaining its original color, but rather has to do with the chemical nature of the replacement mineral. (It would be difficult to know or say what the original color might have been.)

What is the source of information that coral is "bleaching" due to pollutants? I have to suppose "bleaching" is a euphemism, rather than an actual action. It might be possible that normally red coral might tend to grow in a more-pale color, due to environmental distress. But "bleaching" suggests the color was red and faded. And that would be surprising.

JDA.