There are 2 ways in which you can try to deal with hot rocks.
1 - If running in discriminate mode, and hitting lots of them with a high tone or mid tone, you can switch over to auto-tune ( all metal). The target signature will be a lot less strong, and in some cases go away completely. Checking hot rocks this way can save you a lot of time digging them, but it also takes a bit of time to switch back and forth.
2 - You can try to ground balance them out. Get one or two and actually bob the coil up and down over them as you perform the ground balance operation. It may help, but when doing this, I suggest you get a few targets that are good ( ie, nickels, dimes, quarters) and may sure the unit still picks them up.
It all depends on what type of targets you are looking for. If you are hunting for gold jewelry, and the hot rocks hit as high tone, then just dig the mid tones. 85% of gold rings hit as mid tone per tests done in the past by several well known CZ users. If you are coin hunting, it's going to be a pain in the butt if they hit as high tone.
As far as iron goes, and hunting with an Excal vz CZ, I agree with the others that the CZ is better in the sense that you can hear the iron. That is my opinion, and I am biased towards CZ's since they are my main untis for the past 20+ years. I have used Excals in the past too, and like them...just partial to CZ's.
When hunting with an Excal in iron laden areas, you HAVE to circle the target and sweep at a lot of different angles when the Excal nulls over iron. And you have to sweep slow. The Excal can sniff out targets in iron, an Excal users with experience know that when the unit nulls, they have to still slow down and investigate if they want to try to get a target hiding amoung the iron.
I use my CZ's always in discriminate mode 0 all the time..regardless of if there is a lot of iron of none at all. I want to hear all targets, and running in 0 also eliminates getting fooled by iron wrap.