Hari bol,
Could someone please explain the rock form of the Lord commonly worshipped by devotees, or direct me to a reliable source of information on this? I seem to remember a story about Krishna turning the goddess Tulsai into a plant and she, in turn, turning Him into a rock. Is this accurate? I've also heard that this incarnation of Krishna is even more merciful than Gaura-Nitai. How could that be possible?
Hare Krishna,
p.
You should read this article by Swami B. G. Narasingha. It will explain a lot of things.
Please don't neglect the other "rock" form of the Lord - Govardhan!
Here is a nice quote from Krsna book, during the lifting of Giri-Govardhan, which describes a bit of this worship.
When everything was complete, Krsna assumed a great transcendental form and declared to the inhabitants of Vrndavana that He was Himself Govardhana Hill in order to convince the devotees that Govardhana Hill and Krsna Himself are identical. Then Krsna began to eat all the food offered there. The identity of Krsna and Govardhana Hill is still honored, and great devotees take rocks from Govardhana Hill and worship them exactly as they worship the Deity of Krsna in the temples. The followers of the Krsna consciousness movement may therefore collect small rocks or pebbles from Govardhana Hill and worship them at home, because this worship is as good as Deity worship.
Also, this form of worship manifested in Gaura-lila when Mahaprabhu gave a Govardhan sila to Raghunath das Goswami, along with a garland of gunja-seeds (the red, white and black bead like pods). Here is a picture of that sila which is still worshipped today at Radha-Gokulananda (is that right?) temple in Vraja.
Philip, here's a site with more than you may want to know about Salagrama-shilas and Govardhana-shilas.