Mint or ombalo (pennyroyal)
Ombalo, the Georgian name for pennyroyal: a small-leaved wild mint with a sharp, camphor-like scent, usually used dried.
Caution: Use pennyroyal only in small culinary amounts, avoid it during pregnancy and never ingest the essential oil, which is toxic.
Pennyroyal (Mentha pulegium) is a low-growing species of mint that Georgians call ombalo and use mostly dried in sauces such as tkemali. Its aroma is noticeably sharper and more pungent than peppermint or spearmint, with a camphor-like, slightly bitter note that survives long cooking. That sharpness is what gives sour plum and herb sauces their characteristic edge, so it is used sparingly. Dried ombalo can be found in Georgian, Russian and Eastern European grocers or by mail order; plants for growing at home are sold by herb nurseries.
Substitute:
Dried peppermint is the usual substitute, also in Georgian households outside Georgia; a pinch of dried thyme brings back some of the pungency. The sharp, camphor-like note of ombalo is lost, and the sauce tastes rounder and fresher but less distinctive.