High Level Alchemy (high alch, or alching) is a non-combat Magic spell used to convert items into coins at the best price a player could sell them for at a specialty store. The amount of coins generated is precisely 60% of the item's value (its non-GE price). Excluding rounding errors, high alchemy produces 50% more coins than low alchemy, and 100% more coins than selling to a non specialized shop.
The cost of runes to cast high level alchemy is 813 coins but is reduced to 408 coins with a Staff of fire, Fire battlestaff, Mystic fire staff, Steam battlestaff, Mystic steam staff, Lava battlestaff, Mystic lava staff, any skeletal/necromancer versions of the above, Mindspike (fire) or a Dragon staff, as these staves supply unlimited fire runes. Alternatively, the Explorer's ring 4 and Radiant alchemist's amulet provides 15 rune free casts of High level alchemy a day, which still grants full XP.
High Level Alchemy is a popular way for mages to train their Magic level, by buying or making valuable items and nature runes, then casting the spell on the items. For example, a player might buy or make yew shieldbows to cast High Level Alchemy on them. Buying them would create profit or loss depending on the item's market price, while making them from scratch would usually create profit. In practice, smartly buying items in bulk from other players usually results in more coins being created than paid for the converted item. This is widely regarded as a major cause of inflation in the game. The wealth gained by the alchemising party is not relevant; the issue is that more currency is being created in player's hands than is being used.
After the Evolution of Combat, training magic with combat is now much cheaper and faster, and the popularity of High Level Alchemy has fallen greatly. However, this seems to have led to a small drop in the price of commonly alchemised items, allowing a decent amount of profit for some items. For a list of such items, see the alchemy market watch.
High (and Low) Alchemy can be cast on noted items, saving much withdrawal time from the bank, simplifying the "click pattern", and allowing the spell to be used in almost any location. Also, it should be noted that High Alchemy always grants 65 experience (6.5 in Dungeoneering), regardless of the rarity or expensiveness of the item you are alchemising. For example, high-alchemising a Rune bar grants no more experience than high-alchemising a Bronze bar.
High Level Alchemy takes 5 game ticks, or 3 seconds, to cast. It can be cast up to exactly 1200 times in one hour which adds up to exactly 78, 000 xp/hr, although in practice many people cast a little less, due to lag and timing inconsistency.