are all common control marks for the precious metal Platinum which is a more expensive metal than gold but an identical piece of jewelry made of Gold would weigh less than Platinum. Also 18k Gold is only 75% Gold, and 25% base metal, where as a piece of Platinum jewelry is usually 100% Platinum or 90% Platinum.
Platinum, because of its superior strength and its beautiful silvery-white sheen with subtle grey undertones, is often the preferred metal of choice for the settings around white Diamonds.
The last mark is a bit of a problem; it's doesn't look like a country mark, or sponsor mark (that I'm aware of). It looks like the Arabic letter "Thal" - its equivalent in English is the letter G the first letter in Tha-hab = Gold. Could be just a coincidence though. Not sure.
