"This is the most expensive diamond."
True but in Japan diamond in katakana was the most used form across media and at stores that I saw.
It's being used to call out one option from multiple others. If you just google "ga vs wa japanese nihonshock", they have a good infographic.
It really doesn't matter much without context.
I think が relates これ more closely to the verb than は. So I feel that if they had used は then I would be less certain that これ /is/ いちばん高いダイヤモンド because, for example, I could just as easily say これは頭がいちばん高いダイヤモンドです meaning its head is the most expensive diamond. Hence why context is important (頭 あたま means head)
Could "kono diamond ga ichiban takai desu" be an acceptable way to say this?
To add on, i think 一番高いダイアモンドはこのです。 would be ok. The most expensive diamond is this one.
I would say no because the subject goes first and the subject is これ not "the most expensive diamond". Your sentence would be translated "The most expensive diamond Is this"
高い is an adjective (specifically an い adjective), so it does not need の to modify a noun.
ex. 小さいダイヤモンド ex. おいしいダイヤモンド (lol)
If it were a な adjective, it would need the な to modify a noun.
ex. きれいなダイヤモンド vs. このダイヤモンドはきれいです。
But の isn't needed!
I tried that too, I feel like it should work? It made me use これが最も高いダイヤモンドです