OP was, at least one subsequent poster was too. As I said, I'm no cores expert so I don't know what's going on with those.
I reckon (based on my own experience) that core rocks include all types of commodities possible depending on the ring type, but the probability of the hotspot commodity it's named after is the highest percentage of them present, even if still low relatively speaking.
For example, in a Bromellite hotspot the chance of coming across a Bromellite core rock is 60% (I made this up ftr), with the remaining 40% being the other core commodities such as Void Opals.
On top of that though apparently the yields in a hotspot are meant to be higher even for laser mining, so I'm always dropping into a hotspot even if the hotspot is named after a commodity I'm not primarily after. Once you stop expecting 100% of the commodity the hotspot is denominated after, frustration levels drop accordingly.