With sapphire or other crystalline cases, the challenge is structural because the creation of cases involves materials that are tough to work with and which could prove brittle. As sapphire-cased watches are rarities – some are even one-offs – there isn’t exactly a wealth of statistics about shattered or cracked cases, any more than there are tales of smashed ceramic cases. Indeed, the lack of horror stories about now-common ceramic bodes well for sapphire, should the latter material prove more accessible. But is it worth it?

As more than one watch brand spokesperson will tell you, sapphire cases are instant talking points, they’re stunning in appearance and they are able to showcase movements in a way that even skeletons cannot do. Naturally, the brands ensure that the movements in sapphire cases are worth showing off: to the best of my knowledge, no-one has yet housed a Unitas workhorse or $2 quartz movement in a crystal enclosure. These are halo watches, and they look fabulous on all but the hairiest of wrists.

What is the appeal of a see-through case? Think of Cinderella’s slippers. We are talking total pellucidity with sapphire housings, although some brands have found clever halfway measures that provide nearly all of the transparency but with less in the way of engineering requirements. Thus, the dozen we’ve assembled here include monoblocs in which the cases were carved from solid, sandwiched construction that encases a sapphire centre section or the reverse – two pieces of sapphire to enclose the movement and frame – and even a transparent case not made of sapphire.

Sapphire’s main challenge is extreme hardness. Like the less-glamorous but equally contemporary ceramic cases, hardness is the cause of susceptibility to chipping, cracking or shattering. Guadalupe explains that sapphire is fragile, and this creates a manufacturing process that is “very long, and very complicated. There can be a lot of breakage, and we need to re-start many times from the raw material to be able to have a perfect shape and colour as the final result.”

All of the brands here have mastered the art of cutting, carving, polishing and finishing sapphire or other crystalline materials to a degree of visible perfection. Some, like Cartier, have used transparent materials for experimental models, while others, such as Bell & Ross, have managed to make watches that are almost affordable. The latter is important because sapphire cases are found in a number of the watches with half-million to a million-plus price tags.

Again, as with Cinderella’s slippers, sapphire-cased watches have a uniquely magical quality. Whether carved from a block or assembled in sections, they shine and they captivate. But unlike Cinderella and her footwear, dropping them and losing them is not an option, Prince Charming notwithstanding.

4N Sapphire Planet

Distinguished by the colours of the numerals, each of 4N’s three Sapphire Planets is a unique piece. The company has produced the sapphire model (which is offered in standard form with rose or white gold, or sandblasted or black DLC titanium cases) with the numbers in orange, blue and green. The all-sapphire, rectangular case seems to be illuminated by the eye-catching colours and the multi-layered mechanism. And what a movement! This is a jumping-hour/jumping-minute design, with a collection of black discs held by slim bridges to create anarchitectural look that begs to be studied.

It’s a novel approach to mechanical digital displays, which have been around since the birth of the wristwatch. 4N’s MVT01/D01 manual movement uses ten discs instead of the usual three, with four discs in a structural cage for the hours, another disc for tens of minutes and five discs for the single minutes. The numbers are oversized at 5.5mm and, therefore, highly legible.

In addition to the rotating discs, which meet in an outlined area in the middle, the view contains the struts and an opportunity to gaze upon twin barrels with coincident gears, which are good for a 237-hour power reserve. Other design details include a Breguet overcoil, a parts count of 514, 78 jewels and a clever, easy-change strap system. Each of the three Sapphire Planet watches is fitted with a crocodile strap with stitching to match the numerals.

Bell & Ross BR X2 Tourbillon Micro-Rotor

Clever, this: the structure of Bell & Ross’s square-cased, BR0 watches lent itself to a novel solution for a sapphire watch. In this instance, it’s a “semi”, because the 42.5mm structure consists of see-through top-and-bottom solid sapphire plates, between which is the movement in a sandwich configuration. From the side, then, you see a strip of steel. Despite the “filling” being the movement in a square steel frame, the effect is no less naked just because the metal work reaches to the case’s edges. It’s perfectly offset by a grey alligator strap.

For those who want something worth seeing, this doesn’t disappoint. Not only is the BR-CAL.380 movement a flying tourbillon, it’s automatic, with a micro-rotor as the name declares. And the underside view is as enticing as the front, because the clean bridges highlight both the back of the tourbillon and the rotor itself. The movement is slightly skeletonised; in combination with the sapphire components and a case thickness of only 8.9mm, this is a slim piece that won’t snag a cuff.

Two other novel elements deserve mention. The first is that this watch is actually water-resistant to 50m. The second is that it has been priced at €59,000, which is low not only for an automatic watch with tourbillon, but for a watch that is predominantly housed in sapphire.

Bulgari Octo Tourbillon Sapphire

Here, too, we have a sandwich, but in reverse: Bulgari’s offering uses a black bezel and bottom to contain a sapphire frame around a fully skeletonised movement that lets through so much light that it nearly challenges Corum’s Golden Bridge for minimalism. This is the kind of watch that ticks so many boxes, you would have a hard time finding someone for whom it doesn’t act like a magnet.

For openers, it’s an Octo, which is a favoured timepiece around these parts, and which most would agree is a modern classic. The look of the Octo is now familiar, and it has joined the Royal Oak, the Nautilus and the Big Bang as a sport/dress watch of assured elegance combined with a frisson of machismo. The rubber-lined alligator leather strap adds to the sporting feel.

Inside this application of the Octo form is the manually wound Calibre BVL 206 with flying tourbillon, sandblasted finish and 11 bridges treated with black DLC. The most striking aspect of the design, given that we are still talking about an Octo, is the highlighting of the bridges with bold, luminescent lime green to serve as indices. The movement possesses a 64-hour power reserve and it’s water-resistant to 50m.

At 44mm, this is a statement watch in size, not just materials, and it’s an impressing 12.5mm thick: this will not go unnoticed. It’s one of those rare watches where the side view is as arresting as the dial or the back.

Cartier ID Two Concept Watch

This dazzled all of those lucky enough to see it, and “lucky” is apt: it’s a prototype that Cartier revealed to the press five years ago, strictly as a test bed for ideas that would filter down through the company’s haute horlogerie offerings. Crucially, the classically round, 42mm transparent case is a key part of the technological achievement, and not just an aesthetic statement as transparency suggests. To put it another way, the designers of the ID Two Concept Watch didn’t choose sapphire, though desiring see-through status, but selected another unrelated material.

For its transparency and well as other properties, Cartier chose to employ Ceramyst – a see-through polycrystalline ceramic. This allowed the company to make the case out of only two parts, which they say reduced gasket length by 48 per cent. This is crucial because ID Two had, among its many goals, the creation of an airless environment for the movement. This material’s nanoparticle gaskets and minimally porous state preserves the vacuum inside the case “for at least ten years”.

This being an experimental design, the rest of ID Two was equally innovative: the airless environment ensured lower friction, enabling the watch to produce 30 percent extra power but consuming half as much, and it delivered 32 days of power reserve. A pity this stays in the lab or the museum, as it’s one of the best-looking see-through watches ever created.

Greubel Forsey Double Tourbillon 30° Technique Sapphire

Yes, this is one of those watches that breaks the million-dollar barrier, but it is not a pièce unique, having been described as “offered to the US market in a limited quantity for a limited time”. Which tells you that at least one brand considers Yanks as partial to see-through watches.

With Greubel Forsey, however, a transparent case is gilding the lily: with the exception of the recent Vision and a few others, there is no shortage of Greubel Forsey watches that are dial-less, glass-backed and with their movements about as exposed as Lady Godiva at full canter. This company knows that its raison d’être is the complexity of its movements, and does nothing to hide them.

For this model, the base is the Double Tourbillon 30° Technique, and it was developed to expose what little remains obscured in the standard models. Its 38.4mm case is machined from a single large sapphire crystal, the company preferring to accept the challenge rather than to assemble it from smaller, separate components. Thus, the task involved the fashioning of the rounded, multi-angular case horns integral with the case.

Inside, the movement places the Double Tourbillon 30° at 6 o’clock, it features four coaxial mainspring barrels at 11 o’clock for a 120-hour power reserve with indicator at 3 o’clock and a small seconds at 9 o’clock. It is, as any transparent watch should be, an eyeful to dazzle anyone who loves the pure tech of horology.

H. Moser & Cie Venturer
Tourbillon Dual Time Sapphire Blue Skeleton

Just because a watch is a one-off, it doesn’t mean there won’t be another. At least, if legendary retailer Laurent Picciotto of Chronopassion wants not one, but two, and badly at that. He was so taken with the Venturer Tourbillon Dual Time Sapphire Skeleton that he acquired it for CHF 1 million, unable to resist a Venturer with skeletonised movement, housed in a 41.5mm case made entirely from sapphire.

Picciotto commissioned the company to create another, but with a twist to distinguish it from the first. Like all Venturers, it features a classically styled case containing an HMC 803 automatic calibre in full view. For the second Sapphire, however, the skeleton movement has been blued, creating a completely different look and attitude. Side by side, they’d make you hum Cole Porter’s Night & Day.

Contrasting beautifully with the blued background are long, tapered rose gold hands, with the second time zone indicated by a red, rhodium-plated hand. A nice touch to aid legibility is the way the hand hides behind the main hour hand when it is not being used to show a second zone. To further break up the preponderance of blue, the main bridge of the tourbillon is also made of red gold, while the view from underneath includes the bidirectional rotor in red gold; this is also engraved and open-worked.

Hautlence Moebius

Another see-through model using sandwich construction, Hautlence’s Moebius is a massive 52x50mm construct consisting of two sapphire plates capturing a frame and movement, with the metal structures made of white or red gold – and there are only eight examples of each. While looking nothing like the sandwiches from Bell & Ross, MB&F and Bulgari on these pages, the Hautlence does form a sub-genre with those timepieces.

From there on, it’s uniquely Hautlence-esque, with a mix of display technologies seen in the non-transparent Vortex Gamma, including a retrograde system for the minutes in the shape of an upside-down “U” that occupies much of the upper half of the dial. To the left is a digital hour display on a tread formed of a 12-link chain, hinged on a pallet system. The hours change over a 3-4 second period, rather than in a jerking jump. The display also includes a power reserve to track the 40-hour run-time of the twin-barrel, automatic in-house movement.

All of this serves the item at the bottom: a bi-axial tourbillon that enjoys “movement kinematics featuring 90° changes of direction by means of special bevelled gears”. It’s the sort of action that you simply do not want to obscure, making it the perfect justification for a transparent case, the skeletal bits of gold obstructing it not in the least. Just so you’ll see exactly where your CHF 240,000 went.

Hublot Big Bang Unico Sapphire Colours

With so many see-through models from which to choose, Hublot presented us with a conundrum… until we reached Baselworld in March. While LaFerrari in solid sapphire is simply breathtaking, and would stand out in any assembly of fine timepieces, the introduction of the 45mm Big Bang Unico Sapphire in a choice of colours was impossible to resist. Just about every transparent watch in this group is crystal clear; the new Hublots add party spirit thanks to 250 being produced in blue and 250 in red.

Yes, one might say that they look like something you’d eat or, if you’re a cynic, that they represent a grown-up manifestation of Swatch’s Jelly Fish. In which case, you’re missing the point of see-through watches and Big Bangs in particular. Big Bangs, starting with the name, are two-fingers-in-the-air timepieces, and they’re the perfect accessories for spoiled rich kids who double-park their Lamborghinis on Bond Street.

What the sapphire case encloses is the Hublot HUB1242 Unico in-house automatic flyback chronograph with column wheel. The movement is on full display, and the Arabic numerals are colour co-ordinated with the case. Thus, the owner has no fear of his or her watch being mistaken for an outside brand’s plastic interpretation of a Big Bang. These rock. If you’ve got the panache to carry them off, go for it.

Jacob & Co. Astronomia Flawless

If you’re going to produce a completely transparent watch, make certain that what’s inside is worth exposing. Jacob & Co. has gone the orrery route, which crams in as much tech-in-motion as any complication. Seriously, are you more captivated by the visual activity of a perpetual calendar or minute repeater than an orrery?

What a feat of miniaturisation! This 40mm timepiece, which  as preceded by the Astronomia Tourbillon and Astronomia Clarity, uses a massive single-piece sapphire housing to protect a truly three-dimensional drama based on the manually wound JCAM16 movement. The functions include hours and minutes, with the floating dial rotating in 20 minute cycles on the central axis.

With the tourbillon at its heart, the watch contains a sphere representing the moon, made from a white diamond with 288 facets, weighing 1 carat and rotating in 60 seconds on two axes. The earth is shown as a magnesium globe rotating in 60 seconds on two axes, with continents in rose gold and oceans in blue lacquer. Lastly, the sun is presented as a yellow diamond with 288 facets, weighing 2.2 carats and rotating on itself. The gravitational tourbillon cage itself rotates on three axes, with the first in 60 seconds, the second in five minutes and the third in 20 minutes.

And if you like the Astronomia Flawless, you’ll love the Astronomia Octopus.

MB&F Horological Machine NO6 SV and Alien Nation

With a sphere at each corner and MB&F’s penchant for science fiction themes, it’s no wonder that the automatic movement of Horological Machine No. 6 looks like the 1960s Moon Buggy from the Bond film Diamonds Are Forever. It deconstructs the way movements are laid out, with a central section containing the train and flying tourbillon, while two of the spheres show hours and minutes and the other pair provide some drama by looking like a couple of turbines.

It begs to be placed on display. The earlier “Space Pirate” incarnation of this watch looked like a spacecraft from a Chris Foss book cover, sinuous, curvy titanium shells with see-through domes to expose the spheres for the time and the turbine effect. By replacing the metalwork with sapphire top and bottom, in a sandwich layout, and most recently with the Alien Nation, which is a complete sapphire case on the watch, all is open for viewing.

This is a massive “statement” watch, taunting your wrist not to disappear under a 51×50mm slab that’s 22.7mm thick. Your cuffs will hate it. But it is so arresting a show, you’ll roll up your sleeves to proclaim its presence, whatever the occasion.

Rebellion 540 Magnum Grand Tourbillon Sapphire

What an eyeful. This is one busy watch, which –in the manner of Richard Mille models and certain Hublot Big Bangs – is designed to suggest automotive engineering because of the company’s involvement with motorsport at the highest levels. This timepiece possesses the visual complexity that a see-through watch demands, the movement filling the 46×56.7×19.6mm case with a bouillabaisse of intricate hardware.

Its 38-jewel REB T-14 movement is completely open-worked, with a 17mm diameter, 60-second tourbillon at 4 o’clock, a roller-driven power reserve indicator at 7 o’clock to track the 14-day power reserve, a chain-driven, arrowhead power reserve indicator at 9 o’clock, presumably to deal with its two barrels which are situated in a series at 9 o’clock and 12 o’clock and much more – not least being a mix of high-tech materials including magnesium, aluminium, carbon fibre and titanium.

Where Rebellion differs from the brands preferring to use a single chunk of sapphire is the case’s multi-part construction. It uses ten separate components machined from sapphire crystal, with compound curves, bevelled edges and anti-reflection treatment, and it still manages to be water resistant to 30m. To complement the look, Rebellion offers a choice of three straps: transparent silicone, white shark-skin and black ostrich leg. Best of all: the hands are huge, so time-telling legibility hasn’t been compromised.

Richard Mille RM 056 Jean Todt

Like Hublot, Richard Mille has form with sapphire cases and there are a few models from which to choose. And like Greubel Forsey, the company has never actually hidden its movements, almost all of which are skeletal, so eliminating any remaining obstructions due to metal casework can only be achieved through the move to sapphire. With the RM 056 Jean Todt, of which only three will be produced, Richard Mille has one of the most complex-looking watches in this round-up, but – like the Rebellion – it is rescued from illegibility thanks to bold hands.

With a suitably bold creation to mark Todt’s 50-year career in motorsport, Mille has rendered the RM 56-02 Sapphire in Todt’s favourite colours so the chapter ring and crown are trimmed in a rich blue, the hands for the various functions are accented in red and the numerals and minute markings are rose gold. Because this is a skeletal split-second chronograph with flyback function, the colours are mandatory in order to provide a contrast with the mechanism.

All of this resides in the signature curved tonneau shape that surely makes case manufacture more difficult. The company says that the case required “no less than 1,000 hours of machining, including 430 hours devoted to grinding and 350 hours of polishing”. When you see it, you’ll agree that it was worth every minute.