The gemstones might be undatable. But, they might be date-able. If they happen to have inclusions of zircon or monazite or any of the other tiny accessory minerals that are routinely used for determining the date of formation of rocks (usually, but not always, from the radioactive decay of Uranium in the mineral to Thorium and then Lead). However, if those inclusions aren't on the surface of the gemstone you couldn't do it without destroying the gem, and it costs a fair bit of money to do that sort of analysis. So even if they do, there isn't much point. But you can safely assume that they are really old as humans count time. :-)