On “No Heat” Sapphires:
A ‘No-Heat’ Sapphire is exquisite and rare.
About 99% of mined sapphires will undergo heat treatment, a common practice in the jewelry trade, and still be termed ‘natural’, given the natural provenance of Earth’s stones. Unless specifically tested and certified to fulfill the strict criteria for a ‘No-Heat’ stone, even expensive and large sapphires could have been heat-treated. The 1% that is considered color or grade ‘perfect’ will not be heat-treated. A ‘No-Heat’ Sapphire is that 1 percent.
(What Does No Heat Sapphire Mean?)