Gold jewelry is strengthened with other alloys, or metal mixtures, to improve durability as pure gold is too soft for daily wear. While 18 karat (18k) is 75% pure gold, 14 karat (14k) is about 58% pure gold. The remainder is made of other metals that are mixed with the gold, such as copper, silver and zinc.
The purity difference mainly reflects on the price of the jewelry - 18k has a higher price point than 14k - but also on durability and color.
While we do not recommend that you use any of your jewelry during workouts, chores (cleaning, washing etc.) or heavier labor and physical activity (please, feel free to read our jewelry care guide); 14k gold is more durable (harder) than 18k because of all of the other metals included. 18k gold is however more pure, and richer in color. So, while 14k gold has a lighter yellow tone, 18k gold has a fuller gold tone. 18k gold is also less likely to cause allergies due to its purity levels.