Manganese Nodules.
Gold.
No. If Jagex decides to make that possible in the future, maybe. But currently you can't mine gold in oceans on Runescape.
Yes, China does indeed mine for ore. It is a huge and resource-rich country. It is currently undergoing an economic boom that is almost unimaginable in scale, so they need all the ore they can find.
mining does not cost money, so no, it does not. But, it can be sold at a high price to the grand exchange The cost of mining gold varies from mine to mine. For example, if the cost to mine it per ounce is $600 per ounce, it is profitable to mine when it trades much above that, but not profitable if it trades at that or at a lower price. At the price increases,it becomes profitable to open or re-open mines that that have a high mining cost-per-ounce.
A resource that can not be replaced once used up, is a definition of a non-renewable resource. For example: once the coal seam in a coal mine has been depleted, the mine is closed down.
Diamonds. Obviously.
Miners mine iron ore because it is a valuable resource used in the production of steel. Steel is crucial for many industries, including construction, manufacturing, and transportation, so there is a high demand for iron ore. Mining iron ore is profitable and supports economic growth.
One can mine coltan ore by extracting tantalum rocks. One can go to Australia, Brazil, Canada, China, Ethiopia and Thailand to mine them. It is a profitable industry.
Most mineral ores are under the surface. Mining minerals can be profitable.
There are a number of coal mines in the US that are quite large and profitable. The vast majority of these mines are located in Wyoming, which has the largest coal extraction rates of any state.
I don't think that you have to mine in South Africa (or anywhere) but many people have found it profitable to do so. If there is mineral wealth, then digging it out leads to economic success.