What do I do with diamonds?

After finishing the quests that take me through the hotel to get to the parking lot, I accumulated a fair number of champagne bottles, which I dutifully turned in for diamonds. Now I've got 8 diamonds.

The diamonds sell well to merchants (about $1500 iirc), but I've held off on selling any, since rare items like that usually have a better use than being sold for cash.

From this question I see that diamonds are needed for a mod later on, which does not surprise me. What mod is it, and how many diamonds will I need?

Can I sell some of my diamonds and still have enough for late game, or should I hoard every single one?

It's actually five different mod's:

• Developer's No. 1
• Developer's No. 2
• Developer's No. 3
• Developer's No. 4
• Developer's No. 666

Each mod requires 5 diamonds to craft, which means you will need 25 diamonds to craft every single one of them. So I would recommend keeping them until you have enough to craft them all.