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.