The Japanese words for calligraphy practice this week are ishi (regular or small stones) and iwa (large solid rocks, boulders, or crags). These are mostly used in everyday speech. A more formal word for rock is ganseki that refers to any kind of rock. The kanji for ganseki is comprised of the kanji for iwa followed by the kanji for ishi. My example for ganseki below should also be used for iwa and ishi.
Here are relatively simple sketches of rocks for practice. In paintings, these may be elements in compositions combined with other things.
For this painting, you may want to refer to the final Zen rock garden photo at the end of the discussion.