Why diamonds is not a money?

Definition of money:
Money is anything that is generally accepted as payment for goods and services and repayment of debts. The main uses of money are as a medium of exchange, a unit of account, and a store of value. Some authors explicitly require money to be a standard of deferred payment.

Diamond can be used as money. However, in view of following deficiency, it can't play the role of money.
1. Production cost is too expensive
2. Value (ie. Unit of account) can't be certified by ordinary people.
3. Increase transaction cost as a medium of exchange (owing to point 2)

Finally, people tend to keep good momey (if diamond was one type of money) and use bad momey (eg. plastic / paper) in the exchange. For example, gold/silver are the medium of exchange in the history. Once bank note or paper momey is accepted as the medium of exchange. People tend keeping the gold / silver themselve.

Diamonds can certainly be used for transaction as long as the other parties
would accept it as payment. The reason diamonds are not used more
commonly is because not everyone has diamonds. And also, diamonds
quality varies. It's not easy for just any person to tell is a 1 carat diamond
worths US$2000 to US$10000. Paper money is easier to value and backed
by the government, much easier as medium for trades.

Money is an instrument for trade. A person puts forth certain efforts to make a product or provide a service to his community and in turn, he recieves
money as a token to trade these for produce; products or services he want.

The most common materials used to make this instrument are paper; plastic;
metals and through electronic transactions, especially nowadays.

Other than these, diamond is too rare a commodity to be used to (ha, ha, or is
it, I get your point, tricky) make such an instrument. Would you make a
violin out of diamond? Of course not. You don't have such a big diamond.
But you can work; study; perform; think; read; write; calculate; engineer;
plan; organize and do lots of things to get your diamond; money or violin or
see the movie you want to see.

However, you are indeed asking a very creative question. To tell you what
diamonds actually are: they're actually carbon molecules arranged in a
tetrahedral configuration after crystallization into a very hard mineral form.
Did you know there are imitation diamonds? They have a special name for it, I think it's zirconium, not sure, didn't think it was important.

By the way, your question should be "Why is diamond not a money
instrument"