Many players think that mining Coal Ore is a good source of money, but in all examples this is false. Mining Coal is bad because there are faster ways to make money and experience within the same skill. The better way is to make money with Iron Ore.
Coal takes a long time to mine and is only worth about 25-30% more than Iron. By mining Iron you will make more money and experience. Skip coal altogether. You will make a maximum 50K per hour with Iron being around 90K.
For those that still wish to mine coal for smelting or some other purpose the best place is the level 60 mine under Falador. Those with only 30 mining would use the barbarian mining spot, though this has far less coal rocks.
If you need Coal ore though, it would be better to mine the Iron, sell the Iron and then buy the Coal. You will end up with more coal per hour this way, but the choice is up to you.
Happy Money Making