Journey-On: "convex disks of metal, either bronze, tin, or silver, that reflected light off their highly polished surfaces."
An artist depicting bronze would have used a palette of ochre, red, yellow, and orange. In depicting tin or silver, an artist would most likely use shades of grey with flecks of colors of surrounding objects being reflected.
The above object shows no reflection of any images or colors surrounding it. The shape is depicted by the placement of the subject's hand. It think the convex shape would be too distorting to be used as a mirror.
Here is a mirror from late 1st century BC and first half of 1st century AD, which would have been the exact time in question of the era this depiction represents.

http://z.about.com/d/arthistory/1/0/8/j/rafl_afa_08_09.jpg
full description can be found here: http://arthistory.about.com/od/from_exhibitions/ig/roman_art_louvre/rafl_afa_08_09.htm
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I do not believe the object in the painting represents any metal substance. I think this is more likely:
from wiki:
The earliest manufactured mirrors were pieces of polished stone such as obsidian, a naturally occurring volcanic glass. Examples of obsidian mirrors found in Anatolia (modern-day Turkey) have been dated to around 6000 BC. Polished stone mirrors from central and south America date from around 2000 BC onwards.
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And while this object may be pretensiously explained as a "mirror" I believe there is a deeper meaning and hidden message here because meterorite worship was part of a known and existing culture even before BCE.
I also believe this object, even if portrayed as a mirror is not what it seems. From what I have read of Caravaggio and the chilling people in his inner circle (places high and low), and from what I have found concerning unusual events that take place in Malta and allegiances, I suspect this painting means much more than simply "a woman putting away her vanity." That is absolutely trite.
Just curious, but how would you describe the expression on Mary's face? What does it say to you?