It's difficult to definitively say which substance is the most brittle, as "brittle" is a relative term and depends on the specific testing conditions. However, here are some contenders for exceptionally brittle materials:

* Glass: This is a classic example. It's known for its ability to shatter into sharp pieces with very little force.

* Ceramic materials: Ceramics, like porcelain and ceramic tiles, are also very brittle and can easily crack or break under stress.

* Cast iron: While iron itself is quite strong, cast iron is extremely brittle due to its internal structure.

* Diamond: While known for its hardness, diamond is also incredibly brittle. This means it can withstand a lot of pressure but will shatter easily if struck a sharp blow.

Other factors to consider:

* Temperature: The brittleness of a material can change drastically with temperature. Some materials become more brittle at low temperatures.

* Rate of loading: Applying force quickly can make a material more brittle than applying it slowly.

Therefore, the "most brittle" substance depends on the context and what criteria you're using to define brittleness.