|You should pick up the necklace and drape it across your hands, as the answer to your question is literally in your hands. There are two compelling sides to this matter.|
On the one hand, again literally, you should ask yourself the following question: is this necklace worth more than peace in your marriage? On the other hand, you worked hard on a major project, your associate rewarded you with a gift that you truly want, and your husband is showing minimal trust, respect, and security by getting upset and insisting you return it.
At the very least, the two of you need to talk more about this, as there could be any number of underlying issues going on here. In your discussion, it might be rather revealing if you reverse roles and let him defend keeping the necklace while you defend his position of returning it.
As for the right answer, that is for the two of you to determine. But it won't happen without some serious discussion. It would be a real shame to let this necklace turn into a choker on your marriage.