Aquamarine and topaz are strikingly similar and can be nearly impossible to tell apart. It is impossible to tell them apart with the naked eye, as both gemstones can be identical in colouration. They do, however, differ in both thermal conductivity and in the amount of light refraction they produce, so with the proper equipment, it is possible to tell the difference between aquamarine and topaz.
Touch the stone to be tested with the tip of the diamond tester.
Turn the diamond tester on.
Read the results of the diamond tester. If the tester registered thermal conductivity, the stone is a topaz. If it didn't, the stone is likely an aquamarine.
Bring the loupe as close to your eye as is comfortable without actually touching your eye.
Look at the stone carefully through the loupe.
Count the number of refraction lines you can see. If you see two refraction lines easily, the stone is a topaz.