What does the English letter on the ring mean?

ADK: It is the brand name (Aidang)
"ADK Aidang" jewelry is the brand of Shenzhen Ganlu Jewelry Jewelry Co., Ltd.

pt: platinum
n990: The content of gold, silver, and platinum in the jewelry

pd: 钯

platinum, silver -white, highly ductantly, except for king water, is not corroded by acid and alkali.

This gold is a member of the platinum. It is silver -white, symbol PD, light than platinum, and is slightly harder than platinum. It is not easy to oxidize and loses luster in normal conditions. When making 时 jewelry, a layer of 铑 is usually plated. The quality of the 价 is relatively light and the value is not strong. It is rarely made internationally to make jewelry. The poke of 是 jewelry is PD.
The merchants on the market currently use platinum and similar appearances to charges "platinum" to deceive consumers with "钯".

Absolutely amazing! ~~~~~~~~~~~~