Diamonds - A Girl's Best Friend??

My husband has decided that I need a bigger diamond for raising such a wonderful son.  I already have a little over a karat.  He took me to the jeweler today to look at stones.  He wasn't happy because he thought the ones they had were too small.

He wants one that is more than 2 karats!!  

Usually, the only jewelry I wear is my wedding/engagement ring.  I am not a big jewelry person.  

The new diamond would be pretty expensive.  Should I let him purchase it?  I have been trying to decide whether I should try to talk him out of it.  It's a lot of money to spend.  If I just tell him no I will hurt his feelings.  He thinks he is doing something really nice for me.

I have been thinking it might be a good investment.  I don't think they lose their value. 

I know if he purchased it for me it would have lots of sentimental value.  I love my ring I have now because he bought it for me.  

So, what do you think?  Do I encourage him or do I try to discourage him?