In Exodus 38:8 (NASB)
Moreover, he made the basin of bronze with its base of bronze, from the mirrors of the serving women who served at the doorway of the tent of meeting.
What did ancient mirrors look like?
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It is possible to make mirrors from several naturally occurring materials such as a number of minerals (like obsidian, as NigelJ pointed out).
These need not delay us here - this question concerns the materials referenced in Ex 38:8 which were bronze mirrors.
ONE of the materials used in ancient mirrors was polished bronze, common in Egypt. For example, https://en.wikipedia.org/wiki/Bronze_mirror
There is another reference to burnished bronze in
Ellicott says this:
The mirrors used in ancient times were not of glass, but of burnished metal. Bronze was the metal ordinarily employed for the purpose, and was in common use in Egypt, where mirrors were bronze plates, round or oval, with a handle, like our fire-screens. The Etruscan women employed similar articles in their toilets, and had them often delicately chased with engravings.
Mirrors, before the invention of glass, were made of polished brass. Pliny says those of brass and tin mixed together were esteemed the best, before those of silver came to be in use. These here mentioned, no doubt, were of the finest kind of brass, and the women who gave them seem to have been eminent for devotion, attending more constantly than others at the place of public worship, which, is here taken notice of to their honour. In the laver these mirrors were either artfully joined together, or else molten down and cast anew; but it is probable the laver was so brightly burnished that the sides of it still served for mirrors, that the priests, when they came to wash, might there see their faces, and so discover the spots to wash them clean.
Here's an example of a bronze mirror