Tell her right away. Mention that you saw her necklace and ask her if it has a religious value to her, then explain why you asked and tell her you hope it's nothing to worry about.
Why?
1) the less you know her, the easier it'll be if everything goes to drat.
2) she won't be angry you hid it from her and you'll never end up in a drama.
That's what I would have done.
The way you talk to her about it though is most important of all. For this, follow Macid and don't make it a big deal.