24k is considered the purest amount of gold - at 99.99% gold content. 24k gold produces a rich, buttery yellow color but is extremely, extremely soft and delicate as no other metals are added to it - thus is not suitable for daily wear.
18k commonly used in fine jewelry. Its alloy is tested as 75% gold, 18k is durable, but still relatively soft enough to experience scratches and changes to its shape or setting from daily wear due to its malleability.
14k is also commonly used in fine jewelry and considered to be 58% pure gold. In the United States, it is the most common karat weight used in fine jewelry and engagement rings, specifically. We use solid 14k gold in most of our jewelry as we find it is suited best for permanent, daily wear and the most active of lifestyles.
10k is considered the most durable - with 41.7% pure gold content. We use 10k for a few of our rings as we value its durability. As 10k as a higher content level of alloyed metals - it is not considered hypoallergenic for everyone.
To create jewelry for daily wear - gold is alloyed with other metals including zinc, copper, cadmium, silver. This alloy is a proprietary mix of metals that is owned by the manufacturer and is not common or public information. Gold stamped with a karat is considered solid. Today, many jewelry companies will advertise solid gold when it is actually only finished or plated in solid gold. There is limited regulation surrounding marketing language so it is very important to read the fine print and understand what you are buying, especially when it comes to solid gold.
When worn daily, gold will naturally patina. When gold is worn next to other pieces of gold - the pieces will patina to the same tone over time. Occasionally, we might experience our skin turning black with interaction to pure gold. No need to fret - this residue easily comes off with a quick clean using soap and water or a wet wipe. This reaction is caused by gold interacting with lotions, oils, moisturizers, etc. It can also be an indicator of low iron levels in the body.