Let the cost of a 2-inch marble be \( x \) dollars. Then, the cost of a 1-inch marble will be \( x - 0.30 \) dollars (since it costs 30 cents less).
Rochelle buys 25 1-inch marbles for a total of $10. Thus, we can write the equation:
\[ 25(x - 0.30) = 10 \]
Now, simplify the equation:
\[ 25x - 7.50 = 10 \]
Next, add 7.50 to both sides:
\[ 25x = 17.50 \]
Now, divide both sides by 25 to find \( x \):
\[ x = \frac{17.50}{25} = 0.70 \]
So the cost of a 2-inch marble is \( \mathbf{0.70} \) dollars.
The answer is $0.70.