The Sigg Thermo Flask Gemstone combines low weight and compact size to fit easily into my smaller rucksacks – useful when on my mountain bike. The bare polished stainless steel outer slips easily back into my pack, whilst the deep embossed channels down the body mean it’s easy to grip with bare hands, gloves and when it’s wet. The stainless cup/lid has a reasonably sharp top edge so although there is no extra surface detailing to help grip it, that top edge aids grips and the lid comes off easily.
Price: $39 / £28 | Weight: 286g (500ml) | Materials: stainless steel, BPA-free plastic, silicone | Features: pouring stopper and insulated cup | Dimensions: 238x68mm (500ml tested) | Sizes: 500ml, 750ml, 1.1L | sigg.com
The cup is plastic-lined for some insulation, but the steel outside does warm from the contents quite quickly. The plastic stopper has a prominently knurled edge and turns easily with one full turn to allow a good flow into your cup from the arrow-marked pouring channels. The flow is quite even with minimal splashing. Cleaning is straightforward, with a wide access into the bottle and the silicone seal on the lid is removable for cleaning.
‘Lab’ performance was excellent with 88°C contents temperature recorded after three hours, 79°C after six hours and 72°C at nine hours. In real-life hill use, this meant that I needed to let the contents cool before sipping
at my first stop, and I still had a hot drink at the end of the day. Construction is to very tight tolerances to get the very narrow vacuum gap, which gives the Gemstone its slim profile.
A heavy drop onto rocks led to multiple deep dents on the base rim,which haven’t affected performance at all. For this level of performance and reliability, the price is good.