18ct and 9ct refer to the purity of gold.
Pure gold is generally considered too soft and vibrant yellow in colour for most jewellery applications. So pure gold is mixed with other metals to give it the properties and colour suitable for jewellery.
These mixtures are called alloys. We also use alloys to get different colours of gold, such as rose and white gold.
We love 18ct gold, as it is perfect for precious jewellery. At 18 out of 24 parts gold, it is predominately gold yet strong and durable enough for your everyday-and-forever jewellery.
At 9 parts out of 24, 9ct gold is mainly made of non-precious metals. It is great for fashion or lifestyle pieces.