Beitzah, Chapter 1, Mishnah 7. This mishnah deals with grinding spices and salt on Yom Tov: to what extent it is permitted, and with what changes from the ordinary weekday manner.
Underlying the whole discussion is the difference between spices and salt: once spices have been ground, they begin to lose their flavor, whereas salt does not deteriorate. It follows that the ability to grind spices close to mealtime is crucial to their quality, while salt could just as well have been crushed before Yom Tov.
The view of Beis Shammai:
"HaTavlin nidochin b'medoch shel etz" - although crushing is permitted on Yom Tov, a change from the weekday manner is required. The pestle must be made of wood rather than stone, since stone was the usual implement and wood the unusual one. This single change suffices, and the mortar itself may be of stone.
"V'hamelach bapach uv'etz haparur" - with salt, which is not much impaired by being ground in advance, the ruling is not as lenient, and two changes are required: using an earthenware jug, and using a wooden spoon to do the crushing. Some commentators understand that either one of the two suffices, but most understand that according to Beis Shammai both changes are required together.
The view of Beis Hillel:
"HaTavlin nidochin k'darkan b'medoch shel even" - for spices there is no need for a different type of implement as a change, since spices are impaired by being ground in advance, and grinding them on Yom Tov is a genuine food need required for preparing the meal.
"V'hamelach b'medoch shel etz" - with salt, which really should have been prepared before Yom Tov, there is a certain concern, and therefore one who crushes it on Yom Tov is required to use a pestle made of wood.
In summary: the mishnah distinguishes between spices and salt according to how much is lost by preparing them in advance. According to Beis Shammai, one change is required for spices (a wooden pestle) and two changes for salt (a jug and a wooden spoon); according to Beis Hillel, spices are ground in the usual way with a stone pestle, while salt is ground with a wooden pestle.