Casio G-Shock GA-2100 GA-2100-1A1«CasiOak»

The storyWhy this reference matters

In August 2019 Casio launched a G-Shock in Japan that was 11.8 mm thick, and the number was the news. The trick is called Carbon Core Guard: the core of the case is carbon-fibre-reinforced resin, stiff enough to let Casio use less material while keeping the usual 200 metres and shock resistance. Inside sits the 5611 module, ana-digi, with real hands and a small LCD window at 4:30.

Nobody planned what happened next. The octagonal bezel, the flat case and the asymmetric dial had people calling it the CasiOak, after the Audemars Piguet Royal Oak, and the nickname swallowed the catalogue name whole. The GA-2100-1A1 sold out and ran waiting lists across half the world, and within a few years it had become one of the best-selling G-Shocks ever made. Time and Tide made it their most-read review of 2021.

The irony is that the octagon doesn't come from Geneva. Casio says the shape comes from the DW-5000C of 1983, the first G-Shock, redrawn for an ana-digi layout. It is the same trip back to the source the square had already made, only with hands this time: central hours and minutes, a retrograde day hand at nine o'clock and the display at 4:30, which holds seconds, date, stopwatch, timer, world time and alarms.

SpecificationsThe numbers, in one table

IdentificationHow to tell it apart

The -1A1 is the all-black one: grey hands and black markers on a black dial. Perfectly legible in bright sun and a good deal less so by late afternoon. The GA-2100-1A, launched alongside it, has white hands and light grey markers, and the -4A is the red one. This is the mix-up that happens most often in shops, because they share module, case and measurements — only the palette changes.

Watch out for the lume, because there is a catch. The -1A1 is precisely the version that goes without Neobrite on the hands. There is some luminous material, but far weaker than on the rest of the series. Buy the black one expecting to read it at night and you will be disappointed; that is what the twin LED lights are for, and those do work well.

Then came the cousins, and they are worth keeping straight. The GM-2100 swaps the resin bezel for steel; the GA-B2100 adds solar charging and Bluetooth; the GMB2100 is steel throughout. All share the silhouette, so look at the bezel and check whether there is a solar cell under the display. The letters at the end of the reference (ER, DR, CR, JF) only say which market it was packed for: the watch is the same.

CollectingWhat collectors check

This is a watch to wear, not to shelve, though it has its quirks. The resin bezel picks up scuffs, and on the black one it is not the scratches that show so much as the printing wearing away: the letters around the bezel fade off and there is no elegant way to bring them back. Look at the corners of the octagon, where the edge goes first.

Check the display and the hands before you pay. The 5611 module includes a hand-position adjustment function precisely because a hard knock can shift the hands out of step with the digital time. If the hour hand and the LCD disagree, you can correct it on the watch itself, but it is worth knowing why it happened. Try all four buttons too, set into the profile of the case and punished by sweat.

The strap is quick-release and it is the part most often replaced, so the condition of the original matters little. What does matter is knowing whether the watch has been got at: there is an enormous aftermarket in bezels, straps and adapters for the GA-2100. A customised example is worth less than an untouched one if what you want is the -1A1 as it left the factory.

Common questionsWhat people ask us about this reference

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Close relativesThe ones it gets confused with