It depends on what feed you use. Layer feeds have added calcium to be fed to just layers.
I free feed a 20% protein flock raiser crumble to all ages and genders of my confined flock, and a separate free feed container of oyster shell mixed with dried and crushed chicken egg shell for the layers.
Much easier, IMO, to buy one feed and offer the shell...and the higher protein balances out with the 8% scratch grains, greens, meal worms, veggie and meat scraps.
I'm feeding them organic, non-GMO, layer crumbles & they have the run of our 2 acres. Plus they get mealworms, organic carrots & spinach everyday & tomatoes out of our garden when I have some ready. I also give them black oil sunflower seeds & a couple handful of rice most night as a going to bed snack. BTW, haven't had any more weird eggs - that's been the only one. Guess I'll add the oyster shells if I get any more. They are young pullets & haven't been laying all that long so hope it was just a 1 time thing. Thanks for the help everyone.