Question: What is the both the world's tallest stone structure and the world's tallest obelisk?

Answer: The Washington Monument.

Standing 555 1/2 feet tall the monument is the tallest structure in Washington D.C.  Construction of the monument began in 1848 but was not completed until 1884. It officially opened on October 9, 1888. Upon completion, it became the world's tallest structure, a title held until completion of the Eiffel Tower.

The monument was originally capped with a 100-ounce aluminum tipped lightning-rod.  The tip was the largest single piece of aluminum cast at the time.  The total cost of the monument was $1,187,710.

Some facts:

    * Total height of monument: 555 feet, 5.12 inches
    * Height from lobby to observation level: 500 feet
    * Width at base of monument: 55 ft 1 1⁄2 inches
    * Width at top of shaft: 34 ft 5 inches
    * Thickness of monument walls at base: 15 feet
    * Thickness of monument walls at observation level: 18 inches
    * Total weight of monument: 82,421 tons
    * Total number of blocks in monument: 36,491
    * Sway of monument in 30-mile-per-hour wind: 0.125 inches