Gold coils have, like almost all other coils, a copper thread that is wound around the shaft. The copper is released in low quantities and makes sperm sterile. And so, women who use a Cu-coil rarely have ectopic pregnancies. After a few years, the copper thread gradually disintegrates and breaks in a number of places. A metal core, made of gold or silver, that is incorporated into the copper thread prevents this. Whether the metal core is made of gold or silver is of no consequence, since both are equally effective and differ only in price.
Coil made of plastic and wound round with copper, with a blue retrieval thread.