Pinching jewelry: How do I handle my mother stealing items?

I have not seen these suggestions in a long time. Put a black area rug in front of your door. Those with Dementia think its a hole and won't cross it. Another suggestion is put a curtain over the door the same color of the wall.

Get a lock on the door or get a safe and lock your things away.

Why do we leap to assumptions like ‘M has dementia’? OP has no profile and no previous posts, so we know absolutely nothing about this. Perhaps M wants to check if it is “fine gold, diamonds, or costume jewelry”, and if it's expensive, where the money is coming from.

Get a small heavy inexpensive safe with a combination lock. If your closet can be locked, put the safe in there, put the jewelry in the safe, and lock the closet. Then lock your room door.

Consider buying lots of cheap costume jewelry at garage sales or thrift stores. Let mom steal that. Keep putting it back in your room so she'll always have a ready stash.

Or - think of it! - don't live with mom!

Depending on the actual value of your jewelry (fine gold, diamonds, or costume jewelry), I'd get a "keyed interior lock" on your bedroom door. Changing the door handle to a keyed lock (not a bathroom type "passage lock") is not that hard to do, and you get 2 keys with it. You can buy one at Home Depot for less than $20. The holes are already drilled in your existing door, all you need is a 2 thin screwdrivers (Phillips and flat edge).

A bathroom "passage lock" is easily opened with a thin screwdriver from the outside, by poking a screwdriver into a small hole underneath the outside door handle. You want a "keyed interior lock" that has a key slot visible from the outside. Mom may be clever enough to open a passage lock.

I would also go thru your Mom's bag and remove any of your jewelry she's already taken. I wouldn't scold her about it (due to dementia) but I definitely would install prevention for my personal things. 14kt Gold is running $90 a gram these days. Mom could flush a ring down a toilet, or hide something never to be found that could be $300-$500 item, or very sentimental to you.
Good Luck!

Restrict access to anything you don’t want touched

We know nothing about you or your mother, apart from the fact that you are still living with her. You might well be a teenager, just with a curious mother. Please complete your profile and provide more information.

Is she living in your house, or is she living in yours? Either way, put a lock on your door.

is this her only “off” behavior, or is she having other issues?

How do you handle it? By educating yourself more about this horrific disease of dementia, so you are better prepared for what is ahead and you won't be so surprised by your mothers actions.
Your mothers brain is now permanently broken and will never get better, only worse, and she cannot help the behaviors she's now exhibiting, so perhaps put a lock on your door or use the one that's already on it.
Or it may be time to have your mother placed in the appropriate facility where she'll be looked after and you won't have to worry about her coming in and taking your things.

Your mother has dementia, that's why she's taking your jewelry. It's part of the condition for many people because they've lost their filters for knowing right from wrong anymore. Keep your door locked?