Chair Man
If "phallic symbol" is a vaguely long rod-shaped object that can extend and/or shoot things, then how could we picture a "yonic symbol"? Some kind of mouth?
MAX POWER KILL JEEEEEEEEWWWWW
the fan is a line that opens into a triangle
World's Toughest Milkman
A sideways eye/mouth shape, like two joined parenthesis '()' is sometimes used. This has led people to suggest that Peter Jackson's version of Sauron in The Lord of the Rings is a yonic symbol, but this is clearly nonsense (though possibly amusing nonsense).
Also, pretty much anything with a hole in it, especially rocks and carved wood.
More abstractly, tunnels and caves and portals can serve as yonic symbols. Some creators can make a simple door into a yonic symbol.
Speaking words of fandom: let it squee, let it squee.
Chair Man
Oh, that reminds me, the Japanese has the saying "the sword and its sheath" for a phallic symbol and a yonic one.
MAX POWER KILL JEEEEEEEEWWWWW
I was watching a series with many phallic symbols and visual metaphors, and I started wondering if there were yonic counterparts. I thought about how I would do it and a hand fan came to my mind and actually noticed that there was a woman with a hand fan between her legs just in the first episode. So, what are some stock yonic symbols? Is the fan among them?