Radioactive dating corals
Only the surface of the formation contains live polyps, so after drilling, the coral formation should continue to grow in the shallows, unharmed and without interruption.
’” When the coral slices were put in the X-ray scanner, a distinctive series of light and dark growth rings became visible, reflecting the density of the calcium carbonate that made up the coral skeleton.
One can think of ways to correct for this in an hourglass: One could make a mark on the outside of the glass where the sand level started from and then repeat the interval with a stopwatch in the other hand to calibrate it.
Or if one is clever she or he could examine the hourglass' shape and determine what fraction of all the sand was at the top to start with.
By knowing how long it takes all of the sand to fall, one could determine how long the time interval was.
Similarly, there are good ways to tell quite precisely how much of the daughter product was already in the rock when it cooled and hardened.