"What kind of playing surface should a pool table have?" It’s a common question asked by people shopping for a pool table. The Billiard Congress of America (BCA) Equipment Specifications dictate the following (bold added for emphasis):
The playing surface must be capable, either by its own strength or a combination of its strength and that of the table base frame, of maintaining an overall flatness within ± .020” lengthwise and ± .010” across the width. Further, this surface should have an additional deflection not to exceed .030” when loaded with a concentrated static force of 200 pounds at its center. All slate joints must be in the same plane within .005” after leveling and shimming. The bed must be covered with a billiard fabric, the major portion of which is made of wool, with proper tension to avoid unwanted ball roll-off. It is recommended that professional tournament size tables have a three-piece set of slate with a minimum thickness of 1” and a wooden frame of at least ¾” attached to the slate. BCA will sanction tournament play on home and coin-operated tables with one-piece slate providing the Play Area requirements are met. All playing surfaces must be solidly secured to base frame with screws or bolts.
So what is slate? Slate is a metamorphic rock, meaning it morphed or changed its original shale-like sedimentary rock form into its current form. The original sedimentary rock was subjected to heat and pressure, causing it to change dramatically. The sedimentary rock was originally composed of clay or volcanic ash, the crystals of which it’s composed changed through low-grade regional metamorphism, like the collision of two continental plates.
Because of slate's low water absorption index, it is resistant to frost damage. This property makes slate a good choice for tombstones, notably President Kennedy’s. More widespread, it’s used for roof tiles, of course the aforementioned pool table playing surface, and until recently, chalkboards.
Slate is mined all over the world. Slate is mined in Wales and Cornwall in the United Kingdom since as far back as the twelfth century. Elsewhere in Europe, slate is mined in parts of France, Belgium, northern Italy, Portugal, Spain and Germany. The second-largest producer of slate is Brazil. In North America it’s mined in eastern Newfoundland, Pennsylvania, Virginia, Vermont and New York. Slate is also found in the Arctic. Even though China holds the largest slate deposits on the planet, exports only began within the last twenty years because of the poor infrastructure in those areas.
What makes slate such an excellent surface for billiard and pool table is its exceptional stability. Slate has two lines of breakability: cleavage and grain. The ability to cleave or cut slate into thin sheets makes it ideal for use as an affordable and unrivaled, precision playing surface. Granite can be used, but because of its relative hardness, is more expensive to cut & finish; it's just not practical.