TL;DR Gold filled components consist of thick layer of real gold, heat bonded to a base metal core. These pieces have many of the benefits of real gold without the price tag. It doesn't flake, peel or tarnish and is safe for sensitive skin and people with metal allergies. Gold filled pieces are also very durable and will last decades.
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Gold filled items are also known as 'rolled gold' due to the process used to create them, in which a solid gold sheet is rolled around a base metal. Heat and pressure is then used to create a permanent bond of the two metals.
This nice thick gold layer means a gold filled piece can withstand decades of daily wear without flaking, peeling or tarnishing. It also means that a gold filled piece acts a lot like solid gold when in contact with the skin, one of the best benefits being that it's hypoallergenic making it safe for people with sensitive skin and metal allergies.
For comparison to qualify as gold filled the gold layer must be at least least 2.5 microns thick, where as on a gold plated piece the gold layer can be as thin as 0.02 microns thick.
The 14kt part of the name refers to the purity of the gold used. The term "karat" (kt) designates the amount of gold found in a piece of jewellery. 14 karat gold is considered durable enough to last a lifetime with good care and is used in 90% of engagement rings for this reason.
I hope this helps you to better understand the term gold filled and why I use it as standard for all my hook and hoop earrings.