For whatever reason, our Sun and Moon appear to be the same size (from our perspective, which is really what Astrology is about) so I like to use either the same size stone, or at least a little bigger stone than the other planets for the Moon.
But, in actuality, from a real, size to size comparison, our moon is not so big, since both Mercury and Pluto, the smallest planets, are larger in actual size.
Depending on where the moon appeared in your chart, compared to the Sun's position, we will know what your natal Moon phase was/is. This can be coded into the natal chart jewelry in a variety of ways.
http://www.llewellyn.com/journal/article/2354 this article has some interesting revelations about how the natal moon phase influences your relationships.
Moon Phase earrings - the phase, degree, and sign your moon is in, as well as any planets that are in the same sign, are reflected in these drop-style earrings. Can be paired with an Ascendant Earring, to make a pair of astrologically symbolic earrings.