Heirloom RubyHeirloom Ruby is a blend of 55% unmercerized cotton and 45% mercerized cotton. This is the first time we've sampled mercerized cotton, and it's just lovely! It's a bit thinner at 275 GSM and the mercerized cotton adds a bit more glide than the unmercerized does. This fantastic review by April & October compares Ruby to Sapphire, in case you'd like to know how it compares to the other cotton Heirloom wraps. Due to the higher cost of mercerized cotton