It looks like art deco design. The clasp does sometimes help tell the age how it's made, the setting for the stone too. Find marking. Is it gold, plated, or gold wash or other. Does a magnet stick to it?

I haven't found any markings on it, using an 8x lupe.

It does not respond to a magnet and it is plated, though gold or brass I don't know.

By the general design ISTM to be related to some organization, though I don't recognize any symbols.

Please post some clear, high-resolution photos.

I have some antique jewelry which I inherited from my mother and grandmother. I also was a serious collector of vintage jewelry in the past.

In my opinion - based on the items I inherited from my mother and grandmother - some of which had similar tassel designs, I would say this is either late Victorian or early Edwardian - i.e. late 19th or early 20th century. It was not atypical for jewelry from this era not to be made with pure gold unless it was fine jewelry.

At all times, there was a market for women who couldn't afford the "real thing" and typically such less expensive jewelry might also include semi precious stones which were relatively inexpensive at that time.

And materials used for inexpensive jewelry could vary depending on the era. For example, during WW II, base metals were in short supply because of the need for the war effort so there sterling silver jewelry, often plated over the silver, were made for the costume jewelry market. These jewelry have a very distinctive appearance - large brooches are most common - often with a single large stone of some kind with a geometric or floral motif.

Here's a close-up. The late 19th and early 20th century dating sounds right as it may have been something my MIL inherited.

That's a beautiful necklace! I used to deal with vintage jewelry up until 4 or 5 years ago. This type of chain is called "bookchain" and is probably Victorian as mentioned earlier. It's also probably rolled gold, ie "gold-filled."