For mechanical-watch enthusiasts who obsess over a movement’s intricately layered architecture of gears, wheels, and bridges, a transparent sapphire-crystal case is the ultimate frame for a compelling kinetic sculpture.

Today’s watches are generally topped with a scratch-resistant sapphire crystal, and many are fitted with a sapphire case back for a peek into the mechanism. But some brands are crafting entire cases out of the transparent material, so you can see the mechanics from all sides and angles.

Synthetic sapphire crystal is made by crystalizing pure aluminum oxide at extreme temperatures to achieve a material chemically identical to natural sapphire gems. The material measures a 9 on the Mohs scale for hardness, compared with diamond, at 10.

Due to its extreme hardness, sapphire crystal is very difficult to machine and polish, requiring diamond-cutting tools. Producing a full case is costly and therefore often reserved for highly technical complications and limited editions. In addition, sapphire weighs less than titanium, so even voluminous cases can remain light.

Bovet’s Récital 26 Brainstorm Chapter One (US$318,600) presented an extraordinary engineering challenge due to its unorthodox asymmetrical sloped case, inspired by an old-fashioned writing desk

“A curved middle had never been crafted in sapphire,” says Bovet owner Pascal Raffy in a statement. It required the development of new machining and polishing processes. “Such a profile is much more complex to manufacture, and it doesn’t tolerate the slightest imperfection.”

The panoramic case houses a mesmerizing patented double-face flying tourbillon with a variable inertia balance for heightened precision. At 12 o’clock, where the case is thickest, the cutaway hours-and-minutes dial has a three-dimensional moon phase, engraved to mimic the lunar surface and set against a night sky composed of sparkly aventurine glass.

Skeleton-watch specialist Armin Strom also cased a highly technical watch in sapphire crystal this year. The Masterpiece 1 Dual Time Resonance Sapphire (US$268,000), limited to eight pieces, combines two completely independent time indications powered by separate movements in resonance, a phenomenon which naturally synchronizes the oscillations of both for extreme precision. Armin Strom spent three years developing its patented Resonance Clutch Spring that connects the two movements. The watch functions as a GMT (a reference to Greenwich Mean Time), a second time zone, or as a timer or countdown. Plus, each time display has an independent power reserve and a 24-hour indication.

“This new sapphire case sets the perfect stage for the detailed work and passion we put into the Dual Time Resonance,” says Claude Greisler, Armin Strom’s co-founder, in a statement. The transparency “doesn’t allow for any mistakes and demands a finish on the highest level, which goes in line with our philosophy.”

A pioneer in the specialty, Hublot has offered sapphire-crystal cases for years. In 2017, it accomplished the feat of creating large colored panes of sapphire, which has allowed it to produce translucent cases in shades of smoky black, red, blue, and now yellow in this year’s Spirit of Big Bang Yellow Sapphire (US$106,000). In an industry first, Hublot created the sunny hue by infusing the aluminum oxide with copper.

Limited to 100 pieces, the 42mm Spirit of Big Bang Yellow Sapphire dramatically showcases its engine, a Hublot-modified and finished skeletonized version of the legendary Zenith El Primero automatic chronograph movement, which marks its 50th anniversary this year.