Interesting!
Basically, the ore value will increase over time, but the value will increase slower and slower over time. You will always see some growth over time; there is no fixed max limit of ore value.
(Feel free to ignore this next part if it doesn't make sense.)
A "logarithm" is the inverse of an exponent. A graph of a logarithm is an exponential graph reflected over y=x. (The graph attached is a graph of a logarithm)
log (10 ^ x) = x
(End the technical section)
I hope this makes some amount of sense!