Answer: The rock is 2000785 years old.
Explanation:
Expression for rate law for first order kinetics is given by:

where,
k = rate constant
t = age of sample
a = initial amount of the reactant = 800
x = amount of daughter isotope = 600
a - x = amount left after decay process = amount of parent isotope = (800-600) = 200
Half life is the amount of time taken by a radioactive material to decay to half of its original value.




The rock is 2000785 years old.
Natural, it is not man made.
Because one might have other minerals in it as well