It’s mostly materials.
Fine jewelry is usually composed of rare gemstones–diamonds, rubies, sapphires. There is a grey area as to which are included, but a fair line to draw is “are they valuable enough to be graded.” (grading is basically a ranking by certain criteria. For a diamond, it’s color, clarity, cut and carat weight)
Fashion jewelry is made of less expensive stones and man-made stones. For example, quartz, turquoise, swarovski crystals, cubic zirconia. These are common and inexpensive enough that you usually wouldn’t be upset to find they don’t come with any paper certification of their authenticity.
Fine jewelry also uses expensive precious metals: gold, white gold, platinum, etc.
Fashion jewelry generally uses precious metal plating over a base metal. It can also use less expensive metals like sterling silver, brass, copper, etc.
It’s important to say that Fine vs Fashion does not mean Nice vs Cheap, or Durable vs Flimsy. It’s a little lazy to say that, and it certainly casts aspersions on fashion jewelry makers who take great pride in their work. Either class of jewelry can be built to last, built to hand down to your grandkids.
One thing about fashion jewelry: since it’s cheaper, it’s easier to produce. This means fashion jewelry tends to follow trends more quickly. Many Hollywood actresses wear quite a bit of fashion jewelry because it is so much easier for designers to try new things. As handmade jewelry makers know, you are a lot more careful when you are working with 24k gold.
Thanks for reading. Below, I have a few photos of vintage 1950s and 60s fashion jewelry, courtesy of GlitzUK.