Answer
The index of refraction of topaz is $1.62$
Work Step by Step
We can find the index of refraction of topaz:
$n = \frac{c}{v}$
$n = \frac{3.0\times 10^8~m/s}{1.85\times 10^8~m/s}$
$n = 1.62$
The index of refraction of topaz is $1.62$.