>"Nacre" is often used for apertural and overglazing material. I
>should say that I BELIEVE I am correct in saying that nacre is
>actually pearly material, and has a distinct crystalline structure
>which refracts light to produce the pearly appearance.

In its technical use, nacre refers specifically to mother of pearl (and pearl).  It has a distinctive microstructure, found only in certain mollusks with aragonite inner shell layers.  The structure is relatively strong and flexible, but if it does break it tends to break catastrophically.  Other shell structures common in mollusks are generally easier to break but cracks do not propagate as much.

Popular usage applies the term nacre to the inner shell layer (including deposition of inner-like material on the outside) of any mollusk, but this is inaccurate in the technical sense.

    Dr. David Campbell
    Old Seashells
    University of Alabama
    Biodiversity & Systematics
    Dept. Biological Sciences
    Box 870345
    Tuscaloosa, AL  35487-0345 USA
    [log in to unmask]

That is Uncle Joe, taken in the masonic regalia of a Grand Exalted Periwinkle of the Mystic Order of Whelks-P.G. Wodehouse, Romance at Droitgate Spa