You've seen the Facebook Advertising for Gold Fill Jewellery - what's the hype about?

Nothing really, Gold Fill is a piece of costume jewellery with a bit of gold on the outside.

It's not fine jewellery, it is costume, as the base metal, usually brass, is not a Noble Metal.  Having just read this, and the last time you may have heard  or learned about the Periodic Table was way back when, you are asking - "What is a Noble Metal?"

Definition from Britannica - 

"Noble Metal, any of several metallic chemical elements that have outstanding resistance to oxidation, even at high temperatures; the grouping is not strictly defined but usually is considered to include rhenium, ruthenium, rhodium, palladium, silver, osmium, iridium, platinum and gold  i.e., the metals of groups VIIb, VIII, and Ib of the second and third transition series of the periodic table.  Mercury and copper are sometimes included as noble metals.

Silver and gold, which with copper (often called the coinage metals), and platinum, iridium, and palladium comprise the so-called precious metals, which are used in jewellery."

Note that brass is a composite material of copper and zinc (here is Australia) but can contain other metals when made overseas., and will oxidise (blacken) with wear and leave a black mark on your skin.

So Gold Fill is brass or even a copper core, with a layer of Gold over the top.  But isn't Gold Plated the same? 

Here's a chart to help explain it:

To summarize, while gold plated, gold fill, and vermeil jewellery can look beautiful, they may not be as long-wearing or waterproof as some other types of jewellery due to their construction and materials. In contrast, solid gold jewellery (whether pure gold or mixed with other metals for durability) tends to be more resistant to wear, tarnish, and water damage. It's a more stable and reliable option for long-term wear and exposure to various conditions.

 If you're looking for jewellery that you can wear every day and in different environments, solid gold or solid sterling silver pieces are a better choice. They are less likely to tarnish or lose their appearance over time, making them more suitable for long-wearing and exposure to water.