I kept a jewelry because it's my favorite stone, not because my ex gave it to me. Why is it upsetting my bf so much that I get rid of it? ?

Why did you announce that the ring was from your ex in the first place?  

Because you made an issue of it. He sees it as a reminder of your ex.

if its saphire he probally thought it means something related to woman's heart or something. just tell him u like it its pretty to look at. Keep it in a case with other stuff

Because he is jealous. was that not what you wanted?what was the purpose of telling your boyfriend that you got this jewelry from your ex if not an attempt to make him jealous? most people don't point out to their boyfriend every piece of jewelry they were given by an ex

It's a link to the memories you have of your ex.  You're reminded of him every time you wear it.  It's also a reminder to your boyfriend that you were once in love before and he feels threatened by that.