The Collaborative International Dictionary of English v.0.48:

Limestone \Lime"stone`\ (l[imac]m"st[=o]n`), n.
   A rock consisting chiefly of calcium carbonate or carbonate
   of lime. It sometimes contains also magnesium carbonate, and
   is then called magnesian or dolomitic limestone. Crystalline
   limestone is called marble.
   [1913 Webster]

WordNet (r) 3.0 (2006):

limestone
    n 1: a sedimentary rock consisting mainly of calcium that was
         deposited by the remains of marine animals

U.S. Gazetteer Counties (2000):

Limestone -- U.S. County in Alabama
   Population (2000):    65676
   Housing Units (2000): 26897
   Land area (2000):     568.049396 sq. miles (1471.241120 sq. km)
   Water area (2000):    39.031297 sq. miles (101.090592 sq. km)
   Total area (2000):    607.080693 sq. miles (1572.331712 sq. km)
   Located within:       Alabama (AL), FIPS 01
   Location:             34.807445 N, 86.974946 W
   Headwords:
    Limestone
    Limestone, AL
    Limestone County
    Limestone County, AL


U.S. Gazetteer Counties (2000):

Limestone -- U.S. County in Texas
   Population (2000):    22051
   Housing Units (2000): 9725
   Land area (2000):     908.877553 sq. miles (2353.981956 sq. km)
   Water area (2000):    24.272692 sq. miles (62.865980 sq. km)
   Total area (2000):    933.150245 sq. miles (2416.847936 sq. km)
   Located within:       Texas (TX), FIPS 48
   Location:             31.575449 N, 96.551636 W
   Headwords:
    Limestone
    Limestone, TX
    Limestone County
    Limestone County, TX


U.S. Gazetteer Places (2000):

Limestone, NY -- U.S. village in New York
   Population (2000):    411
   Housing Units (2000): 188
   Land area (2000):     1.628996 sq. miles (4.219080 sq. km)
   Water area (2000):    0.013843 sq. miles (0.035852 sq. km)
   Total area (2000):    1.642839 sq. miles (4.254932 sq. km)
   FIPS code:            42378
   Located within:       New York (NY), FIPS 36
   Location:             42.026029 N, 78.632983 W
   ZIP Codes (1990):     14753
   Note: some ZIP codes may be omitted esp. for suburbs.
   Headwords:
    Limestone, NY
    Limestone


U.S. Gazetteer Places (2000):

Limestone, OK -- U.S. Census Designated Place in Oklahoma
   Population (2000):    745
   Housing Units (2000): 258
   Land area (2000):     3.154987 sq. miles (8.171378 sq. km)
   Water area (2000):    0.000000 sq. miles (0.000000 sq. km)
   Total area (2000):    3.154987 sq. miles (8.171378 sq. km)
   FIPS code:            43057
   Located within:       Oklahoma (OK), FIPS 40
   Location:             36.311296 N, 95.755040 W
   ZIP Codes (1990):
   Note: some ZIP codes may be omitted esp. for suburbs.
   Headwords:
    Limestone, OK
    Limestone


U.S. Gazetteer Places (2000):

Limestone, ME -- U.S. Census Designated Place in Maine
   Population (2000):    1453
   Housing Units (2000): 495
   Land area (2000):     2.608670 sq. miles (6.756425 sq. km)
   Water area (2000):    0.000000 sq. miles (0.000000 sq. km)
   Total area (2000):    2.608670 sq. miles (6.756425 sq. km)
   FIPS code:            39265
   Located within:       Maine (ME), FIPS 23
   Location:             46.912580 N, 67.826376 W
   ZIP Codes (1990):     04750
   Note: some ZIP codes may be omitted esp. for suburbs.
   Headwords:
    Limestone, ME
    Limestone