How to cook scallops that will prevent them from becoming too rubbery?

It sounds like maybe you cooked them too long. Searing the perfect scallop requires a few steps, and some thought, but it is not difficult. ...and, a cast iron pan is very effective, so don't be put off by previous comments. 1. Use only dry-pack scallops. 2. Before searing, make sure that they are a dry as possible, by placing them on paper towel and drying all sides (you want them to sear, not steam). 3. Find a pan that is large enough so that the scallops are not touching. Otherwise, use more than one pan. 4. Get said pan very hot (I use cast iron with great success, but try other pans to see what you like). 5. Add clarified butter (so that the milk solids don't burn). 6. Add scallops. 7. Don't move the scallops! 8. When you notice a brown crust forming after about a minute, flip the scallops. 9. Cook another minute on side 2. (Alternately, cook most of the way on side one, creating a deep brown crust...so perhaps a minute and a half to two minutes...then flip for 30 seconds) 10. Remove from pan to a paper towel. Bottom line: Dry, hot, fast.

Scallops, like any mollusk, are fast cooking. A cast-iron skillet, which holds heat, is the wrong piece of equipment.

Take them off the heat before you think they are done. By the time you get them out of the pan, onto a plate, and then served, they'll be fully cooked.

First of all, the quality of the scallops is important. Cheap scallops are often treated with sodium tripolyphosphate. These scallops store excessive water and it's very, very difficult to get a good result with them, as you need way too much time and heat to cook them.

Second, your scallops should feel dry when touched. If they don't, dry them with a paper towel, salt them slightly, let them rest on a paper towel for ten minutes, then dry them again.

Third, for a decent result, you have no other choice than to carefully monitor and test the doneness. Fish and shellfish become dry and rubbery at 140°F / 60°C. So the inside of your scallops should be somewhere between 130°F / 55°C and 140°F / 60°C max. This is a fairly narrow margin and this is why undercooked or overcooked fish is very common, even in restaurants.

Experienced chefs can determine the doneness with their fingers; you can peer into a small incision whether the interior is still translucent. It's better to have one scallop used as test object than a whole pan of ruined scallops.

There is nothing wrong with cast iron skillets - you just cannot reduce the heat in time if the pan turns out to be too hot; but unless you end up with burned scallops, this is not your problem. Your problem might be that you are used to a timing that is correct for less heat and that you rely on a feeling, instead of actual monitoring and checking the doneness.

On Masterchef a few months back, Marcus Wareing shared a tip to avoid overcooking scallops.

Rather than tipping them all into the frying pan/ skillet at once, place them one at a time around the edge of the pan, starting at the 12 o'clock position and working clockwise.

Work in the same order when it is time to turn them over or remove them from the pan. By using this "first in first out" method you should get a reliable, even cooking time for each scallop. I've tried this several times now and it works well.

the advice that the cast-iron skillet is the wrong piece of equipment is right, however, it doesn't offer an alternative. Scallops should be seared in a heavy pan. The heavy base will diffuse the heat to all parts of the base, thus giving a uniform cooking area. Try a stainless steel or copper pan, and keep the heat down a little.