Not JMG, but I had some thoughts: For the coin, in terms of trad astro-magic, you may be able to draw a small sun sigil on the back or something like that, and consecrate it at the right time, but it doesn't need to be melted down. However, there are less intensive ways of imbuing it with solar energy without actually calling in a celestial spirit. You could, for instance, consecrate it every Sunday at dawn (best while the Sun is in good dignity, and he's not bad in Sag for fire triplicity ruler) 6 Sundays in a row with solar incense, and wear it in a yellow pouch over your heart, maybe with some Solar herbs.
In Jyotish, diamonds are a Venusian stone and are very powerful - you don't want to use a diamond if you have Venus as the lord of a bad house or in a bad house (I've heard of things like people going into organ failure after putting on a diamond because their Venus was in the 8th) so I would consult a Vedic astrologer before wearing a high quality diamond (especially in a setting where it touches your skin) If you have a good Venus that needs some help though, a diamond can be fantastic.
In Western Astro Magic, diamond is also the stone for Algol, which is one of Warnock's more popular talismans for protection.