In recent years, sapphire crystal has moved on from being used as a luxury replacement to plexiglass on watch cases. Today, we see entire watches case being made out of sapphire. Even though it is a rarity given the cost and effort to mill an entire case out of sapphire, several brands have experimented with such cases.

The obvious benefit is that these watches allow users a glimpse of the movement in ways that they would have never experienced before. The flip side, of course, is that these watches are prohibitively expensive thanks to the high cost of production. Here are a few examples.

Bell & Ross BR-X1 Chronograph Tourbillon Sapphire

The sapphire case is made up of nine parts

The watch case of this Bell & Ross BR-X1 version is entirely made of sapphire crystal. The case is made up of nine parts – the case middle, back, bezel, crystal and four bumpers for the sides, all held together by vertical and lateral screws.

The 45 mm sapphire case is paired with a translucent rubber strap inlaid with metallic finish woven Kevlar. The watch is powered by the BR-CAL.285 that is used in the other BR-X1 chronograph tourbillon models. The movement features open-worked bridges and a base plate. It boasts a power reserve of four days and uses a single-button chronograph with a flying tourbillon regulator.

Hublot Big Bang Unico Sapphire

This watch uses a transparent resin dial

The Hublot Big Bang Unico Sapphire, launched at Baselworld 2016, is an edition of 500 pieces and represents the brand’s first large-scale series of watches with cases cut from pure sapphire blocks. The middle, bezel, and back of the 45-mm-diameter case are cut from blocks of sapphire; only the screws, crown (which is overmolded with silicon) and the watch’s deployant buckle, are made from titanium.

The openworked dial is made of transparent resin and shows off the watch’s movement, Hublot’s in-house Unico Caliber HUB1242, which includes an integrated flyback chronograph function, with the column wheel and double clutch both visible on the dial side. The price? $57,900

MB&F Horological Machine No. 6 SV

Six sapphire domes sit on this case

The “SV” in the name of MB&F’s new watch, the MB&F Horological Machine No. 6 SV, stands for “sapphire vision.” The watch is a completely transparent version of the company’s HM 6 Space Pirate of 2014. There are sapphire plates on the top and bottom of the watch.

Glued to these plates are nine sapphire domes; five on the front of the watch and four on the back. Four of the domes cover the rotating minutes and hours indications, which are made of aluminum machined to the thinness of paper.

Four more cover two turbines, driven by the winding rotor (visible on the back of the watch) via a gear train designed to multiply the number of rotations. The purpose of the turbines is to slow the spinning of the rotor, via increased air friction, in case it starts spinning too quickly. This reduces wear on the rotor. The center dome covers the flying tourbillon. The watch is a limited edition, with 10 pieces in rose gold ($368,000) and 10 in platinum ($398,000).

Richard Mille Tourbillon RM 56-02 Sapphire

The sapphire case is made of three parts

The transparent sapphire case of this Richard Mille watch is made up of three parts, each milled and ground from blocks of solid sapphire. It has a hardness of 1,800 Vickers and is virtually scratch-resistant to any material short of diamond.

Sapphire is also used in the movement, the RM 56-02, namely the winding barrel bridge, the center bridges, and even the tourbillon. The “see-through” aesthetic extends to the partially translucent strap, created exclusively for Richard Mille from a proprietary Aerospace Nano material whose silky, organic texture complements the ergonomic design of the case. The watch, limited to just 10 pieces, is priced at $2,020,000.

Rebellion 540 Magnum Tourbillon Sapphire

It took 99 working days to create this case

Rebellion is an independent brand that’s known to make watches that go against the traditional norms of watchmaking. It is best known for making the T-1000, a watch with a vertical gear train and a power reserve of 1,000 hours.

According to the brand, it took 47,600 minutes (99 working days) of diamond grinding to extract the eight elements that make up the case of the 540 Magnum Tourbillon from specially cut sapphire blocks.

The crown is made from sapphire crystal too, but the winding stem and case screws use steel and titanium. To further aid transparency, all the gaskets used are made from clear silicone. The case is paired with a transparent silicone rubber strap.

The 540 Magnum Tourbillon Sapphire is powered by the Caliber REB-T14 movement which is distinguished by a large flying tourbillon that’s 17.2 mm in diameter. The flying tourbillon cage used in the sapphire version is made from an aluminum alloy in the sapphire version rather than magnesium. The watch costs about $1.9 million.

Greubel Forsey Double Tourbillon 30º Technique Sapphire

The case is machined from a single block of sapphire

Limited to just eight pieces and exclusive to the United States, the Greubel Forsey Double Tourbillon 30º Technique Sapphire features a solid sapphire case that has been carefully machined from a single block of sapphire crystal.

The aim is to offer full visual access to the multi-level movement, with its two tourbillons, from all possible angles. The case is 49.95 mm and 17.15 mm thick, just a bit larger than those of its predecessors in rose gold, white gold, platinum and titanium. The crown, engraved with the GF logo, is also made of sapphire. The watch is priced at approximately $1.1. million.