by duplicating? Only know of two ways to do it now.
Sending a minecart chest with diamonds down a rail through a nether portal and taking the diamond blocks out before it goes into the portal. Turning the blocks back into diamonds before you enter the portal too.
AND
blowing up a chest with diamond blocks and taking the blocks out at the last second.
Sending a minecart chest with diamonds down a rail through a nether portal and taking the diamond blocks out before it goes into the portal. Turning the blocks back into diamonds before you enter the portal too.
AND
blowing up a chest with diamond blocks and taking the blocks out at the last second.