Here’s a selection of granite sundials. Granite is the stone of choice because of strength, resistance to freeze and thaw, and ability to maintain a polished surface through years of outdoor service.
The polished sundial surface is “mechanically” induced with diamond embedded polishing pads.
These sundials are laid out using mathematic formulas that allow accurate time indication for a specific location on Earth. Their time will vary from mechanical clock time for a few reasons.
Daylight savings time will obviously cause a discrepancy between real sun time and time indicated on a clock.
Also, the apparent speed of the sun traversing the sky changes during the year (actually the Earth is rotating making the sun appear to travel east to west). The sun appears “fast” or “slow” in an annual cycle. Because of this, time on a sundial can vary by as much as 16 minutes from what you see on a clock.