Turning a human body into a diamond
Could a human body be compressed into a diamond?
The original question was "How big would I be if I fell under a tectonic plate and was compressed into a diamond?"
A human body would be difficult to compress into a diamond since diamonds are made of pure carbon and bodies are made of many substances - mostly water - and the carbon in a body exists within a variety of chemical compounds.
The density of diamond is 3.53 grams per cubic centimeter; the density of the human body is about 1.0 gram per cubic centimeter; so a body compressed to diamond density would be 28% of its current size.
Tectonic plates move very slowly – centimeters per century at most. Also, the motion is not smooth and continuous at any given location - plate boundaries are motionless relative to each other most of the time and then move in sudden jolts that cause earthquakes.
The SEED article What Are Diamonds Made Of? describes organic carbon being dragged down deep into the Earth because of plate tectonics. A body falling between plates would be ground into small pieces. It is possible that carbon from part of that body might reappear millions of years later within a diamond.