The ‘Diamond Ring’ within Cygnus X, south-west of the DR21 ridge, stands out as a prominent, nearly circular structure in infrared (IR) and far-infrared (FIR) emission, spanning approximately 6 pc in diameter. It is enclosed by clumpy molecular clouds seen in CO lines and contains an H II region, visible in cm emission. It resembles a classical H II region associated with an expanding bubble seen routinely in the 158 μm line of ionised carbon ([C II ]). However, our recent observations utilising the Stratospheric Observatory for Far-Infrared Astronomy (SOFIA) under the FEEDBACK program for the spectrally resolved [C II ] line have revealed a slightly tilted ring with a mass of ~10 ³ M _⊙ , advancing at a velocity of ~1.3 km s ⁻¹ . The bulk emission of the gas has a line-of-sight (LOS) velocity around −2 km s ⁻¹ . The [C II ] data revealed that the ‘Diamond’ of the Diamond Ring is an unrelated, dense gas clump at a LOS velocity of ~7 km s ⁻¹ . The driving source, which is also responsible for powering the associated H II region, is a B0.5e star, classified by our IR spectroscopy. This observation marks the first instance where we observe only a slowly expanding ring of [C II ] emission and not an expanding 3D shell. We suggest that the H II region (along with its associated [C II ] bubble), initially formed by a massive star, expanded outwards from a flat slab of molecular gas nearly in the plane of the sky. Presently, the [C II ] ring is confined by the swept-up gas of the slab, while the parts of the shell moving in the directions perpendicular to the shell along the LOS have dissipated, resulting in a notable decrease in the expansion of the remaining ring. This scenario is supported by dedicated simulations that trace the evolution of the [C II ] bubble. Our observations support the scenario of H II region evolution in ‘flat’ molecular clouds, reported earlier in the literature. In this geometry, we propose that the Diamond Ring represents the terminal phase of an expanding [C II ] bubble driven by stellar winds and thermal pressure.

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