When refrigerated, how do amphetamine crystals appear?

Amphetamine crystals typically appear as colorless needles when refrigerated. This crystalline structure can often form bundles due to the way the molecules align during the crystallization process. The presence of these slender, clear needle-like formations is a key characteristic used to identify amphetamines under microscopic analysis.

In the context of the other options, distinct brown clumped needles and large rosettes with a reddish color do not accurately represent amphetamine crystals, as these descriptions point to different substances or impurities. Whetstone-shaped crystals are also not typical of amphetamines; rather, they could belong to other chemical compounds. Thus, the correct identification of amphetamine crystals as colorless needles reinforces the importance of recognizing specific crystallization patterns in the identification of controlled substances.