You're not supposed to use cast iron skillets with ridges on the bottom of the pan where it comes into contact with the burner. Also, the iron can discolor the glass, or scratch the glass. I have a glasstop and I use cast iron. All of my skillets are smooth bottomed and I don't care if it discolors the glass top. I don't like the glasstop anyway, it came with the house and I'm not impressed with it.

Sometimes the burners of glasstops can run so hot they crack the cast iron pans. You can buy "diffusers" to put on your burners to prevent this. I've found that cast iron heats so efficiently I have to cook at low temperatures or I burn off my seasoning.

I cook with tomatoes in my cast iron, too. I read this blurb a while ago when I was researching how to season my pans

Acidic items like tomato sauces will be darker from iron leaching out, but many people with iron deficiencies do this for extra iron in their diet.

Never store acidic products in cast iron. In fact, never ever use your cast iron pots for storing any foods.

from The Irreplaceable Cast Iron Skillet

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