Heat treatment exposes a gemstone to controlled high temperatures to improve or create its colour and clarity. For sapphire and ruby it is permanent, widely accepted in the trade and common across the market. It does not make a stone synthetic: the stone was mined from the earth, then treated.
Most sapphire on the market is heated because the process can improve colour and clarity permanently. That allows more mined material to reach the market with the appearance buyers want. An unheated sapphire with comparable colour and clarity is scarcer, so it generally carries a higher price.
Heated does not mean fake, and unheated does not automatically mean better. Colour, clarity, cut, origin, size and condition still matter. Treatment status is one part of the description and one part of the price.
Teals sells treated and untreated stones. When treatment is known, the lot names it plainly. Heat, oil, resin, irradiation and filling are different interventions, so the description uses the relevant term rather than grouping them together.
When a stone has not been tested, uncertainty is part of the description. Not independently tested for treatment. Assume standard trade treatment for the species unless a report says otherwise.
Natural on Teals means earth-mined, not unenhanced. A natural stone may still have been treated after mining. Naming treatment where it is known, and uncertainty where it is not, gives a buyer the facts needed to compare one stone with another.