>Has anyone seen or collected old crystallized coral?  I think the crystals are called calcite, but I'm not too familiar with this field.  I've been finding old coral pieces that have "yellowed" and started forming crystal points, and I was wondering if this is a common find in the fossil shell pits.<

There are a couple of processes of fossilization that can produce such results. This applies particularly to coral (not the Paleozoic ones, though) and to most mollusks because of their aragonite skeletons.

On the one hand, aragonite can spontaneously recrystallize into calcite.  This will not produce crystals obvious to the naked eye, but it will change the structure.  If stained by other ions, this may affect the color as well.  Both aragonite and calcite can be replaced with silica or other minerals as well.

However, in order to get a nice geode-like bunch of crystals, the shell or coral must first partially or entirely dissolve away, leaving a hole in the sediment or rock.  Crystals of calcite, quartz, or other minerals can then grow in the space.  This produces a replica or cast of the original fossil. Also, an originally hollow shell can have crystals growing inside it.

Such casts are common at some localities but rare or absent elsewhere, depending on the depth and chemistry of the groundwater, the nature of the matrix, and other diagenetic factors.

    Dr. David Campbell
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