Traditionally, heirloom jewellery was gold. It may have been thought of as a hedge against inflation for the baby as she (and sometimes he) grew up.
These days heirloom jewellery can be sterling silver, often with inset gemstones and often engraved, as in a baby ID bracelet.
Gold bracelets for babies are still considered heirloom gifts and once outgrown, are stored carefully and brought out with pride to show friends and family as she grows up.
If you want to give an item to a newborn, you can't go past a gold baby bracelet as the gift that will go on giving long after it's outgrown. This will be the gift she remembers from her childhood and the one that's captured in all her photos and baby portraits.