Question: What is the best curling iron to use that won't damage hair?
Answer: Look for irons that combine tourmaline technology with porcelain or ceramic design. Tourmaline is a natural crystal commonly used in hot irons and blow-dryers because of its negative ionic charge (which eliminates frizz and closes your hair cuticle) and infrared heat (which provides faster styling with less damage), while porcelain and ceramic irons distribute heat evenly on rods, meaning there are no "hot spots" that can burn hair and cause damage.
Some good ones to try include:
T3 Bespoke Labs Medium Duality Iron (it works as a curling and straightening iron)
Infiniti by Conair Tourmaline Ceramic Curling Iron
CHI Turbo Professional Ceramic Curling Iron
Also, don't forget to protect your hair before curling it. The idea is to put a barrier between your hair and direct heat, so any styling product or leave-in conditioner is better than nothing at all, but products like: Matrix Iron Smoother, Nexxus Heat Protexx, L'Oréal Nature's Therapy Heat Control multitask by providing heat-damage protection and hold for your style.