what is the correct formulation

It's right

AnonymousThe diamond loses 20% of its value if it doesn't have a certificate.

The diamond depreciates by 20% if it doesn't have a certificate.

I'd use the first one. This is true immediately.
I think of depreciation as taking place over time.
You could also say, "The diamond has lost 20% of its value if it doesn't have a

Hey. Mathematically depreciates is more clear ;-) The value after depreciation is the value when you talk. Therefore it could be understood that such 20% is calculated according the LOWER value, and not to the original value. The second wording (depreciates) is more "precise". You could laugh about the argument, but some newspapers, marketing materials, politicians, etc. use this trick often.

I would state this in a completely different, unambiguous, way:
A diamond without a certificate is worth 80% of a similar diamond with a certificate.

"The diamond loses... if it doesn't have..." is completely wrong because it mixes tenses.

You could, if you want to put it this way around, say:
"A diamond is worth 20% less without a certificate"
Or, if you start with

gorgeous explanation.

i was watching about these topics in some pages surfacing on the web, they were extremely entertained as this page because when everyone doesn´t have anything to do, everyone intent to find something to read or pass the time in free times.

i love to pass my free time fixing problems in some works that i have learning english and they are as 30 or more works, but i have always foun

Anonymousi love to pass my free time fixing problems

I too.
These days, I find the trick is not to exacerbate them.