If it is in fact from the 1920s, it’s not cubic zirconia because that wasn’t commercially produced until another fifty years later. But I wouldn’t be surprised if they’re paste jewels, which are cut glass designed to look like gemstones. That being said, being paste doesn’t necessarily make it worthless. But there’s a huge difference between 3-4 figures and the 6 figures they’re imagining.

First step is to look at guides online about distinguishing paste from diamond; LW can likely determine themselves what it is. If it actually seems like it might be a diamond, then get it appraised, because there’s no point twisting yourself into knots preemptively over something that is likely to be a far smaller payout than you expect.