Sharpstone

Sharpstone, also known as novaculite, was a quartz variety that occurred in various colors.

Description
This stone was usually gritty and its major use was for sharpening. Occasionally the grit was fine enough to make large cabochon ornamental stones. Sharpstone's hardness made it difficult to polish to full luster. A typical stone had a base value of 10 gp.

Powers
This stone could be used to increase the area of effect of a spell with a corresponding decrease in damage (or duration if the spell did not have a damage effect). Any size or amount of sharpstone could be used and was consumed in the casting.