"This is the most expensive diamond."
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)
No, が states that the previous word is the new information. は states the following phrase is the new information (thus the previous word is brought up to a topic). In both cases, the new information is being stressed.
The following is the typical question/answer situation.
Question: どれが一番高いダイヤモンドですか。 Which is the most expensive diamond?
Answer これが一番高いダイヤモンドです。This is the most expensive diamond.
Question: これは何ですか。 What is this?
Answer: これは一番高いダイヤモンドです。This is the most expensive diamond.
The two sentences are different.
In saying "this diamond is the most expensive," we already know "this" is a diamond and adds a property "expensive" to it.
In saying "this is the most expensive diamond," we do not know "this" is a diamond, and first we assert that "this" is a diamond, and then add the expensive property to it in one go.
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.