In the text Exodus 28:15-21 consideration is given to part of the High Priest's attire--specifically the square cut of cloth that held precious stones of various colors and degrees of translucence.
Now if one could ask, "What is the perfect Gem?" most likely the response would NOT be the KIND of gem, but more about how that particular gem was cut to reflect and refract the light. This seems to be the case with those precious stones in the breastplate of the High Priest. You see, the Urim and Thummim were God's divining tools of that day to give reply to Aaron and were placed behind the gems. Urim and Thummim are the Hebrew terms that are interpreted as "Lights" and "Perfection." As the perfect lights shone through the gems, it would be the brilliance and glory of each stone to refract and reflect a particular aspect of that light.
Christian, we are those precious stones just as the names of God's people were etched on them long ago. How do you show forth the true light of God? What peculiar hint of beauty do you attribute to God's nature? And how could you possibly shine out that light if you were not part of the gathered gems (the church) with the true Light in the midst? Don't let your light fade, your value decrease, by isolating yourselves from God's assortment of precious stones of the church where the perfect Light resides.