Maybe not a rock exactly I just saw a post about the revival of a 32.000 years old plant. The article said scientists found a seed somewhere burried and we’re able to find out it’s age. How is that possible? The same thing for bones and other ancient stuff.
There are numerous different methods, which often need to be used in tandem, or to calibrate or verify each other.
For young objects, you sometimes have historical records to go on, for example, you might know that a well-known tree has been there since a certain date because people kept describing it in books. Or you might know that a certain patch of rock was produced by a particular volcanic eruption.
Then you have tree rings – many plants show seasonal growth patterns that you can use to estimate how old they are.
You can also use knowledge about rock formations to work out how the ages of different objects relate to each other. For example, if two different kinds of fossils are always found in the same layers of rock, that implies they are roughly the same age.
You can also use evolutionary knowledge, for example, if you already know that a certain kind of fossil gradually changed in a particular way over time, you can examine a specific fossil and estimate its age.
And then there are various radioactive dating methods. These are all based around situations in which an object would have had a predictable amount of a certain radioactive isotope when it was created, so we can see how much radioactive decay there has been and use this to date the object. The most well-known of these is radioactive carbon dating (but there are many others). Most samples that contain carbon will have a mixture of carbon-12 and carbon-13, which are both stable, as well as some carbon-14, which gradually decays into nitrogen-14. However, the level of carbon-14 in the atmosphere keeps getting topped up due to cosmic rays, which sometimes collide with nitrogen-14 nuclei and cause them to turn into carbon-14. So anything that continuously exchanges carbon with the atmosphere (e.g. an animal or plant) should have its carbon-14 topped up while it’s alive, but once it is dead, the level will start decreasing.
Most living things contain Carbon. There is a “special” type of carbon that breaks down over time. The amount of special carbon in a living thing is known and how long it takes to break down is known. With that you can take something very old, see how much special carbon is left and then do some math to get a rough idea of how old it is. That is what Carbon Dating is.
Also if it was buried, then they might be able to tell from the geology of where it was. Or a combination of both.
The method/isotopes used vary depending on what exactly you’re trying to date, but the core concept is generally the same. Radioactive elements/isotopes decay at a certain rate into other elements. Based on how much of the initial radioactive isotope (parent) there is compared to how much of the new one (daughter) there is, you can get a pretty solid idea of how old something is. There are certain assumptions and corrections that need to be made depending on which method is used, where it’s found, etc but that’s the gist of it.