Iridium-Reinforced Stone is a block which is completely impervious to all explosions. In testing, when a hollow 3x3 cube of Iridium-Reinforced Stone and an identical cube of Reinforced Stone are created and an activated Nuke is placed inside, the Iridium-Reinforced Stone will completely insulate the environment from the blast and not be destroyed whereas one block of Reinforced Stone will be destroyed and a small portion of the explosion will leak into the environment. In everyday use, the effects are not very noticeable, and Iridium-Reinforced Stone is very expensive for the small amount of benefits that it adds on top of regular Reinforced Stone. However, Iridium-Reinforced Stone is permanent and cannot be blown up by any means, except by a Wither's explosions.
It is interesting to note that Iridium-reinforced Stone can be blown up by a Wither's block destruction mechanics that it uses to reach a player despite its complete invulnerability to all other explosions. Also interesting to note is that if a Nuke is placed and Bedrock is placed on all six of its sides and the same arrangement is also created with Iridium-reinforced Stone and the Nukes are both activated, the explosion that leaks out from both boxes will form the same exact pattern and with the same exact orientation, with most of the explosion leaking towards the south-easterly direction (a possible example of vanilla Minecraft's South-East rule.)