this may or may not be a DC-specific question.
what is a melong, and why would someone want one? by that I mean, what is its function?
Me long literally means "to arouse fire" suggesting its original use.
In general, the mirror is a symbol of rig pa. In the DC, it is worn for the purpose of Guru Yoga, primarily.
The mirrors sold by SS store are made from five metals, and all contain a bit of the original mirror given to ChNN by his guru. For DC people, other mirrors are fine, but in general it is better to use one made by Giorgio since they are made according to Rinpoche's precise specifications.
Other melongs you may see are one's crafted for long life mirrors, which function to summon all the essence of the elements reflected in that mirror. Other mirrors are worn as good luck charms. These mirrors typically are stamped with the srid pa'i ho mandala on the back and worn on one's belt. These are usually made of bell metal.
Prisms are also called "me longs."