It was conversation I was hearing, the free, passionate, witty exchange of studied minds as polished as fine tools.

I was hearing 是定语从句。可以把 It 换成 This/That。

判断方法：如果强调 conversation，必须特指，需加定冠词，最好加指示代词（this, that）才够资格。一个没有特指的名词，何成强调的对象？  定语从句的先行词，可以不特指。此外，强调句的引导词 that ，正规英语不可省略，因为它是强调句的“标志词”。