The antonyms of "de-composable" are "indestructible", "non-biodegradable", and "non-degradable". These words refer to materials that cannot be broken down naturally or decomposed. Plastic, for instance, is non-biodegradable and does not decompose over time; it only breaks down into smaller pieces, causing harm to the environment. The opposite of de-composable implies that the material will remain unchanged for a long time or forever, thus posing a threat to the environment. Natural, biodegradable materials such as wood, leaves, or food waste are de-composable and, therefore, a more sustainable option for manufacturing and consumption, reducing waste, and protecting the planet.