There are two "treasure rooms" in the Hermitage: The Diamond Room highlights jewels and the Gold Room highlights gold pieces, especially Scythian gold. Both require a special ticket and your admittance is with a small group with a guide.
I went in low season - April 2012. There were no line-ups to buy tickets to get in. Once past the ticket booth at the entrance, there is another desk in the lobby where you can buy a scheduled entrance to either of these special exhibits. There are several time slots, with different language guided tour options. I suspect that the outside lines in July would be very busy, just to enter, and that spots would go very fast for the Diamond and Gold Room tickets
I did not get the impression that these tickets can be booked in advance for individuals, but I could be wrong about this. I ended up joining a German language group for the Gold Room tour. Although I didn't understand any of the commentary, I got to stay within 'eyeball' distance of the guide and had enough time to really appreciate the pieces on display.
For the Diamond Room, I was with an English-speaking tour and, frankly, I don't remember any of the guide's commentary at all. Even in low season, I had to be flexible as far as the availability of the slots that I could get.
I spread my visit over two days, by buying a "Two Day Entrance Ticket" online. I had to exchange my voucher at the entrance booth and I think that, in the busy season, there is a shorter line for voucher holders.
It's entirely a matter of preference for choosing one over the other. There was a greater diversity of objects in the Diamond Room, but the Scythian Gold entranced me.
The Gold Room is down a further corridor from the main part of the Hermitage. On my way back, I detoured into a side gallery and was alone in visiting several rooms with very unique items from Altaic nomad burial mounds (Steppes). Stuff I had never come across before - superbly preserved for over 2,000 years plus (fully dressed horse mummies, very fine wool and leather items, etc.). Despite being a history buff, I was completely ignorant of the rich ancient civilizations from this part of the world.