10. Zara collected 4 marbles, which are made of agate, tiger's eye, steel, and amber. She wants to display these 4 marbles in a row on a shelf, but she does not want the steel marble and the tiger's eye marble to be next to each other. How many arrangements are possible?
(A) 6
(B) 8
(C) 12
(D) 18
(E) 24
10. C.

Let $S$ and $T$ represent steel and tiger stone, respectively. First, arrange $S$ and $T$.
Since $S$ and $T$ cannot be adjacent, the only possible arrangements are:
$$
S \square T \square, S \square \square T, \square S \square T,
$$

and the cases where $S$ and $T$ are swapped, giving a total of $3 \times 2$ ways.
The other two positions can be arranged in 2 ways.
Therefore, there are $2 \times 6 = 12$ different arrangements in total.