[Lalwendë]

Gold of course is a metal associated with Melkor so it was the appropriate choice of material. And if you look at how Gold is referenced in the text then it is certainly not portrayed as being dull or 'boring'. Just from the saying "All that glitters is not Gold" we know that Gold is very highly prized. Yes, Mithril is more rare but does that automatically make it more beautiful? Platinum after all is more rare and more expensive than Gold to us, but people still lust for Gold.

As for setting a gem into the One, it would interrupt the flow of the engraving! A circle containing a 'spell' or 'charm' is a hugely powerful thing - note how many Megalithic monuments were constructed in circular form, and this was carried on in the form of the Celtic Cross, the symbol of Christianity set into the unending symbolism of a simple circle. A literal Ring Of Words would not be as effective were it interrupted by a gem stone. The gem would in fact tarnish the magic.