They have stolen a diamond worth one million dollars

1) is correct, 2) is not.

They have stolen one million dollars worth of diamonds.

1) They have stolen a diamond worth one million dollars. 
They have stolen one single diamond worth one million dollars.

2) needs correction:

If it is one diamond:

They have stolen a million dollar diamond.

If they stole a number of diamonds, together worth one million dollars:

Note the position of the apostrophe

They have stolen a million dollars' worth of diamonds.
or
They have stolen one million dollars' worth of diamonds.

if I'm trying to quickly tell a friend what happened


They stole a million dollar diamond !!

To both me and BH, whether 'a' or not, it sounds like, there was this huge stone, worth millions; but when she got it back she found that a million dollars' worth had been cut off and kept

Kinda only kiddin'

I'm sure there's a meanin' where the 'a' is required; but the sentence can be written n'any of 443 different ways,' pending on our nat'l, ur meanin', ur proclivities

..or was that 444