Question about Act II, Scene 9 - help by 6/28?

I am doing a study of this play in preparation for an upcoming performance. I wonder if anyone has any insight into the symbology of the inscription in the silver casket as delivered by Aragon. In my edition it's Act II, Scene 9, lines 63-71. The text is thus:

"The fire seven times tried this;
Seven times tried that judgment is...
etc.

Can anyone elucidate for me the significance of the number seven at this time in history, especially in this passage? Also, please shed some light on the remainder of this inscription. I simply don't understand the significance of what's being said.

I'm not too sure about much of the details, but silver is tested seven times or purified seven times in fire.

I don't believe that the number seven has any significance, but I can't be sure. If it is a biblical reference, then I wouldn't know.....

The two lines after this (Seven times tried that judgment is/That did never choose amiss) could be saying that if his judgement was strong (or tried seven times), he could have chosen right? It's a long shot, but I'm grasping at straws here.

Hope some of this helps!