Coins famous for their silver content gained an important role in the adornment of wearable heritage. Gradually their value shifted from currency value to amuletic significance. Depictions and numerals on the coins were valued highly and as these coins over the centuries became less common or over-expensive, imitations were made and used as amulets.
Sometimes the depictions are accurate imitations or casts of the original coin; sometimes the depictions as well as materials used do not even remotely resemble the original coins they hoped to imitate. In the last category the designer deliberately emphasized those characteristics of the coin that were most significant to him or in his culture. This provides a unique insight into a culture. On some of the Egyptian coins thhe words ‘Medaillon for Egyptian Ladies’ is cast, addressing the respectability of the wearer with the image on the 'coin' resembling the Maria Theresia Thaler.
Some materials also replace originally more precious ones, like clay red beads may replace red coral. Plastic bicycle reflectors may replace carnelian (see here for an example). In this last particular case, the material is different, the apotropaic quality of the color remains and is even enhanced by the reflecting feature.