That’s the question I was asked by a colleague, whilst walking through the foyer of a local university. This product is actually a terrific slate called ‘Silver Gray Quartzite‘ that has a honed/polished finish.
Silver Gray Quartzite slate is often confused with granite – it is shiny (although it does not quite have the same degree of luster) and is typically found in commercial buildings that have high-volume foot traffic. Because of the high quartz content, it is extremely durable and hard wearing, almost on a par with granite.
Unlike most granite, however, which has a very granular composition and appearance, this quartzite slate has the various shades of colors, displayed in attractive waves, adding a further dimension to the unique look. More images can be found here and here.