First, let’s understand what a diamond dust mirror is. No, it is not diamonds, but, it is silver. Basically the silver was melted and used to cover the backside of a piece of glass. When first used it did not coat evenly.
It was discovered that by adding mercury to the mix that the silver would spread evenly across the glass. In this process the mercury evaporated leaving the silver. This process would leave a telltale sign; what appears to be diamond dust.
When dating mirrors, a diamond dust mirror will be pre-1940 and certainly could date well into the mid to late 1800’s.
So, how do you identify one? Just need a loupe for it is easily spotted. Take a look with a loupe at a modern mirror and when you see a diamond dust, you will recognize it instantly.