Spell Stock is used to cast Spells and is labeled in brackets next to the MP cost, under "cost(stock)". The amount of spell stock used to cast a spell is random, but proportional to the MP cost, for example: a spell that costs 10 MP to cast will use about 10 stock, when the spell level is 1. This amount will be reduced as the spell level goes higher. In Elona+, as of version 1.19, the Spell Stock cost has been reduced to 1/3 of what it used to be. As of Elona+ version 1.59 you can pay platinum to a town wizard NPC to practice the spells you know without consuming any spell stock.