A tea warmer is not just great for keeping your brew warm, but also creates a lovely ambience as the flicker of its tea light radiates an inviting glow. Like teapots, tea warmers come in a range of styles and materials like glass, cast iron, metal, wood, and stainless steel. Choosing a tea warmer will mainly come down to what will match your teapot the best as most warmers will do an equally good job of keeping your teapot warm.
As an accessory, the size of the tea warmer should also be taken into account as some might be too small for certain teapots. Larger tea warmers are not usually an issue and will safely hold a small teapot, but the coupling may look mismatched and disrupt your table setting.
The only other thing to consider would be whether the tea warmer has a more enclosed design or not. An enclosed vessel is mainly for the benefit of protecting the tea light flame. However, enclosed warmers may also help in concentrating the heat upwards instead of dispersing it outwards like open warmers may do.
Side note: If you’re looking to keep a cast iron teapot warm, avoid using enclosed hand blown glass tea warmers as they may be too delicate to support the weight of a cast iron teapot. There are solid glass tea warmers that have a more open design, which may suffice for heavier teapots; but in general, fine glass tea warmers should probably be limited to warming just glass teapots.