Alloys are added to gold to make jewelry more durable and customizable. Simply put, an alloy is a base metal that is added to balance the gold out to be used for whatever application is necessary - this also drastically decreases the value of the gold depending on what alloy you decide to use.
Since there are no alloys in it it’s very soft and malleable, not to mention expensive. A lot of jewelers don’t have the skill to work well enough with it so they’ll try and sell you 18k or 22k. But 24k has to be very carefully and meticulously worked on by hand super slow.
So the labor on the work for 24k gold is also a lot more than the others. 24k gold also has a more deeper gold color to it as well. When we work with alloys, depending on what you use it doesn’t hold the deep color of 24k gold and some people are forced to need to use and deep gold polish on top of the gold to match that color.
Gold coins and bullions are 100% pure 24 karat gold. A lot of people opt to buying those as an investment in gold as well! They aren’t jewelry unless a jeweler is asked to use one in a pendant. That way you can keep the bullion or coin as an investment but also have it to wear on your neck.