Silver is a precious metal. Thus, the product you added is fine, not semi-fine jewellery.
Yes, both glass and lab diamonds are manufactured, but it doesn’t mean that a lab diamond is an equivalent to glass.
Semi-fine jewellery is jewellery that is between costume/fashion and fine. Most brands do not call their silver jewellery 'fine', only reserving that category for gold.
Semi-fine can be sterling silver, gold-plated, gold-filled or vermeil.
Nothing wrong with glass, synthetic, lab-grown or composite stones in jewellery, they are not however equivalent to natural stones. OP was considering replacing natural stones with lab-grown which would compromise the vintage piece. Since then, OP has wisely decided against doing it.