Gravel is a common and very effective drainage substrate for terrariums. Small 2-3mm mixed gravel is perfect for mini terrariums for drainage. The base layer is one of the most important layers in your terrarium. It is essential to have this layer for drainage. 🌿If you want to layer your terrarium with different textures and colours, this is also very suitable and it has a lovely natural look. 🌿It's also perfect to use as a decoration on the top layer of your terrarium.