Although we have learned on yesterday’s daf (=page) that stones that broke away from a mountain on their own may remain permitted, even if they were worshipped, stones that were mined and removed from the mountain are understood to have been subject to human intervention, and may, therefore,
become forbidden. The Mishnah on today’s daf teaches that there are three types of rocks –
1. Rocks that were mined for the purpose of serving as a bimus – a base, or foundation for an idol. Such a rock is automatically forbidden.
2. An ordinary rock that was plastered and painted to honor pagan idol worship. If the plaster and paint were removed, the rock would be permitted.
3. An ordinary rock that was used temporarily to support an idol. Once the idol is removed, the rock is permitted.
The bimus, or base, prepared for an idol is worshipped together with the idol and is, therefore, considered like the idol itself according to Rashi. The Ra’avad argues that a rock mined for this purpose is the matzevah that the Torah expressly prohibits (see 16:22), which is why it is forbidden from the moment that it is prepared.
With regard to the case where an ordinary rock was used as a support for an idol, Rashi explains that since this was only temporary and there was no intention to make this rock into a bimus, once the idol was removed the rock reverts to its original status and there is no need to actively negate its use as a pagan object. The Ra’avad argues that even if the idol was worshipped while situated on the rock, since the rock was not mined for this purpose, nor was any change made in the rock to accommodate the idol on it, it is as though the idol had been placed on the floor, and the rock’s status is unaffected.