well improving @Michael's answer
I am very noble.
The noble was the first English gold coin produced in quantity,also can be reference to Nobel metal(gold is noble metal)
I am loved by many people
well, who doesn't love gold
How good were those early days!!
Now,I have almost disappeared from the face of the Earth
maybe referring to increasing consumption of gold and decreasing amount of the gold on earth
Only the Royal can test me, no one else can.
this was my initial guess
well Royals have the utensils made of gold
after much discussion and with the help of hint this is new guess(thanks @Jasen)
Aqua regia, also known as 'Royal Water' a very strong acid (nitric acid + hydrochloric acid) it is used to refine gold, and it can also dissolve gold
People view me as prestige and honor..
I change every day. Recently, I skyrocketed.
gold price changes every day