What do diamonds represent in the bible?

Answer 1

Four times.

Exodus 28:18

And the second row [of the breastplate] shall be an emerald, a sapphire, and a diamond.

Exodus 39:11

And the second row, an emerald, a sapphire, and a diamond.

Jeremiah 17:1

The sin of Judah is written with a pen of iron, and with the point of a diamond: it is graven upon the table of their heart, and upon the horns of your altars;

Ezekiel 28:13

Thou hast been in Eden the garden of God; every precious stone was thy covering, the sardius, topaz, and the diamond, the beryl, the onyx, and the jasper, the sapphire, the emerald, and the carbuncle, and gold: the workmanship of thy tabrets and of thy pipes was prepared in thee in the day that thou wast created.

Answer 2

The same as the represent nowadays - wealth and opulence

Additional Answer:

In Exodus 28 and 39, a diamond is among the jewels on the breastplate of Judgement which had 4 rows each of 3 stones of which in the second row was the diamond. The stones represented the 12 tribes of Israel.

Diamonds were also used as writing tips as in Jeremiah 17:11

Lucifer's crown/covering had a diamond in it as well:

Ezekiel 28:13New King James Version (NKJV)

13 You were in Eden, the garden of God;

Every precious stone was your covering:

The sardius, topaz, and diamond,

Beryl, onyx, and jasper,

Sapphire, turquoise, and emerald with gold.

The workmanship of your timbrels and pipes

Was prepared for you on the day you were created