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The set has a clamp fastening system with a silicone gasket. The lids look like they are made from glass but they are actually plastic. They do not come pre-labelled so can be used for other foodstuffs besides beverages. As the lids are see-through I don't think it is necessary to label them as you can easily see what they hold.
The jars are not of an equal size - but come in a small, medium and a large.
The set is available in black, white and an aqua colour if this colour does not suit.