One of my favourite firms producing costume jewellery has to be Ciner. The firm started in 1892 but did not begin making costume jewellery until the early 1930’s. The pieces are of very good quality – and often look like the real thing. The firm is still in operation. I always buy vintage Ciner if I can find it – but it is quite sought after and difficult to find at a reasonable price. Ciner is still making and their web site has an interesting section on ‘Our Story’ – see it at https://cinerny.com