I have a conference pear tree which has been very abundant this year.
Every now and then you come across a pear that has a hard nodule inside
the fruit. It typically varies in size from a pea to a walnut. I am
curious to know what causes it and hope someone here knows.

Sometimes there is sign of deformation in the fruit near the nodule, so
I was able to predict that the pear in picture number 61 (links below)
would contain one. The nodule is hard enough to deflect the knife, you
can see this one in place in picture 65. There is not usually a hole in
the nodule but this one is near the little "flower" at the bottom of the
fruit and the dark wispy bits (picture 73) are part of that. Nodules are
often hollow like this one (picture 74).

I speculate that it might be a gall, but few nodules have any sign that
an insect might have left. A wilder guess would be that it is a genetic
defect and the tree is trying to make nuts. The truth is probably more
simple than that. Does anyone know?

http://www.mrtlfrm.demon.co.uk/public/conf74s.jpg
http://www.mrtlfrm.demon.co.uk/public/conf73s.jpg
http://www.mrtlfrm.demon.co.uk/public/conf65s.jpg
http://www.mrtlfrm.demon.co.uk/public/conf61s.jpg
--
Ellis Morgan

The yucky hollow bit in the middle of the lump suggests to me that it was insect damage. You didn't find the insect because it grew up and flew away, and the pear healed over the entrance.

You can get hard lumps which sound generally like what you say from Stony Pit Virus, which is quite variable in presentation, but with just one pit per occasional pear, with a rotten cavity in the pit, is not typical of that disease. Stony Pit is incurable so hope it's not that.