Let the cost of a 2-inch marble be \( x \) dollars. Since the 1-inch marbles cost 30 cents less than the 2-inch marbles, the cost of a 1-inch marble will be \( x - 0.30 \) dollars.
Rochelle buys 25 1-inch marbles for a total of $10. Therefore, we can set up the equation:
\[ 25(x - 0.30) = 10 \]
Now, simplify the equation:
\[ 25x - 7.50 = 10 \]
Add 7.50 to both sides:
\[ 25x = 17.50 \]
Now, divide by 25:
\[ x = \frac{17.50}{25} = 0.70 \]
Thus, the cost of a 2-inch marble is \( \boldsymbol{0.70} \) dollars.
The correct answer is \( \mathbf{0.70} \).