Among the most important items RS gold when smithing, leveling or machining is Coal Bag. You can find this item in the Prospector Percy Nugget Shop where you can trade your 100 golden nuggets with this bag. It is able to hold up to 27 (+9 in cape maximum) parts of Coal. It is extremely useful for each of Mining and Smithing as carrying more ores means greater exp or money made. Having 27 slots can be considered an inventory that is a second one (for Coal only). Coal Bag can be upgraded with the Skill Cape which you can get when you reach the 99 level of Smithing
They are dropped by the Ice Queen that resides in White Wolf Mountain. While she is part of Heroes Quest you do not need to start or complete it in order to reach her. All you need is level 50 in Mining skill to get cheap OSRS gold through certain stones on the mountain. Ice Gloves can be very helpful in many scenarios, especially during the quests, however, their main function is in the Blast Furnace.