Diamonds are one of the 31 Rare Resources in the game and available from the beginning. They appear as a bunch of bright blue 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:
Wealth gather rate +20- Raises
Commerce Limit by +10%
If diamonds are available nearby the starting location, they should be connected early, as they provide a fair boost to the otherwise weak initial Wealth production.
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).
Trivia
One of the hardest and most enduring substances known to man, diamonds are also prized for their beauty. Until the European "ages of discovery", the source of diamonds however was limited to India — the Qohinoor or "Mountain of Light", now part of the British Crown Jewels, is a testament to a time when India was the sole supplier of all the world's diamonds.