Most coloured gems in the trade have been treated in some way: heated, oiled, filled, irradiated. Treatment is normal; not saying so is not. Look up a gem's usual treatments and what they mean for care and for the bench, read what each trade code stands for, and check when the US rules require you to disclose it.
Usual treatments are the ones commonly met in the trade for this gem, not a statement about any single stone: only a laboratory can say what has been done to a particular stone.
The American Gem Trade Association's codes, used on invoices and memos across the trade. Each picture is an illustration of the idea, not of a real stone.
The US Federal Trade Commission's Jewelry Guides (16 CFR 23.24) apply to sellers at every level of the trade. Disclosure can be made at the point of sale, before the sale; when the buyer cannot see the product in person (online, catalogues, TV shopping), it belongs in the product description. Other countries have their own rules: check them for each market you sell to.
Treatment codes: American Gem Trade Association, as published in Understanding stone codes (Stuller BenchJeweler, 19 June 2018): N, B, C, D, F, H, HP, I, L, O, R, U, W and the product terms ASBL, SYN and IMIT; definitions summarised in our own words. Disclosure: US Federal Trade Commission, 16 CFR 23.24, Disclosure of treatments to gemstones, read via Cornell LII.
Per-gem treatments and codes, care and plating: the same data as JewelLoop's live gemstone reference table (GIA Gem Encyclopedia and GIA articles on treatments; GIA care and cleaning guides; JewelLoop bench guidance on plating from Stuller and MJSA). Pictures: ray-traced stones (JewelLoop renderer) and drawings; illustrations only.