I began by examining the crack more closely. It was about 6 inches long and seemed to be spreading outward from the center of the sill. I checked to see if the crack was just superficial or if it went all the way through the stone. Fortunately, it was only about 1/4 inch deep, which made the repair process much simpler.
Standard caulk or wood filler will fail. You need products designed for stone and masonry. how to repair cracked stone window sill
Ideally, the repair mortar should be slightly weaker than the original stone. If stress returns, the repair should fail rather than the surrounding stone. A repair that is too hard (e.g., standard Portland cement) will create a "hard spot," directing stress into the surrounding soft stone and causing new spalling. I began by examining the crack more closely
A "deep" approach to repair requires accepting that If the symptom is treated without addressing the etiology (cause), the repair will fail. The primary mechanisms of failure include: Fortunately, it was only about 1/4 inch deep,
This is the aesthetic phase. The mortar must be tooled to match the surrounding texture.
The repair should not be left as a distinct patch. The edges should be feathered slightly to blend with the weathered patina of the sill. Pigments may be added to the mix to match the stone's color, though they should be used sparingly to avoid a "painted" look.