Let the cost of a 2-inch marble be \( x \) dollars. Then, the cost of a 1-inch marble, which is 30 cents less, can be expressed as \( (x - 0.30) \) dollars.
Rochelle buys 25 1-inch marbles and spends a total of $10. We can express this as:
\[ 25 \times (x - 0.30) = 10 \]
Now, we'll simplify and solve for \( x \):
\[ 25x - 7.50 = 10 \]
Adding 7.50 to both sides:
\[ 25x = 17.50 \]
Now, divide both sides by 25:
\[ x = \frac{17.50}{25} = 0.70 \]
Thus, the cost of a 2-inch marble is \( \boxed{0.70} \).