Diamonds are one of the 31 Rare Resources in the game and available from the beginning. They appear as a bunch of their namesake diamond crystals. Like with all rare resources on land, they can be gathered by sending a Merchant from the Market to the resource location. Diamonds provide the following benefits:
The 10% commerce limit increase are based on the commerce limit from the current Commerce Research level. Example: Say, you have researched Commerce 4 (Mercantilism), the commerce limit would be 260. Having Diamonds connected would raise the limit by 26 to 286. Commerce limit bonuses given from wonders (like the +50 Timber/Wealth from the Colossus) are not taken into account when calculating the 10% (thus, in the above example the Colossus would simply increase the limits for Timber and Wealth from 286 to 336).