To charge is to infuse something with energy usually for a specific purpose.
I believe you've got various methods and techniques to do so. While it's all about manipulating your energy and directing it to the item (quarts in your case) specifically, you can also chant something in order to build up more energy which will charge the item in addition to the rest of the grounded energy you've used to charge the item by simple energy manipulation and direction.
There are some ways and various methods to bless ourselves and make different charged items either generally and either for a specific purpose, as I said, which can be represented by any chant that shows what your desire is. You can just simply charge it with positive energies and bless it by different methods you may find. There are simple methods of charming items and charging them with positive energy, by doing that you "bless" the item, and add your own energy so it's matched to your purposeand your own energies. I also highly recommend being positive while you're around the rose quarts in order to keep it cleansed with positive energies. I'd recommend you to look for methods of charging items with positive energy, so you'd be able to officially charge the necklace with your positive energies for protection.
It mostly requires energy manipulation, visualation of floating energy and directed into the item, use of grounded energy, etc. sometimes you also charge such items by simply being positive around them and also by giving them a pecial meaning and obtaining a positive idea about them associated with your intent / purpose, while you don't always know that you've done so.