Keystone refers to a type of limestone

Limestone

Limestone is a sedimentary rock composed largely of the minerals calcite and aragonite, which are different crystal forms of calcium carbonate . Many limestones are composed from skeletal fragments of marine organisms such as coral or foraminifera....

, or coral rag

Coral rag

Coral rag is a rubbly limestone composed of ancient coral reef material. The term also refers to the building blocks quarried from these strata which are an important local building material in areas such as the east African coast and the Caribbean basin .It is also the name of a member — the Coral...

, quarried in the Florida Keys

Florida Keys

The Florida Keys are a coral archipelago in southeast United States. They begin at the southeastern tip of the Florida peninsula, about south of Miami, and extend in a gentle arc south-southwest and then westward to Key West, the westernmost of the inhabited islands, and on to the uninhabited Dry...

, in particular from Windley Key

Windley Key

Windley Key is an island in the upper Florida Keys in Monroe County, Florida, United States.U.S. 1 crosses the key at approximately mile markers 84—85.5, between Plantation Key and Upper Matecumbe Key....

 fossil

Fossil

Fossils are the preserved remains or traces of animals , plants, and other organisms from the remote past...

 quarry, which is now a State Park

Windley Key Fossil Reef Geological State Park

Windley Key Fossil Reef Geological State Park is a Florida State Park located at mile marker 85.5 near Islamorada. It was a former quarry used by Henry Flagler in the early 1900s to help his building of the Overseas Railroad. Following the railroad's completion, it was a source for decorative stone...

 of Florida

Florida

Florida is a state in the southeastern United States, located on the nation's Atlantic and Gulf coasts. It is bordered to the west by the Gulf of Mexico, to the north by Alabama and Georgia and to the east by the Atlantic Ocean. With a population of 18,801,310 as measured by the 2010 census, it...

. The limestone is Pleistocene

Pleistocene

The Pleistocene is the epoch from 2,588,000 to 11,700 years BP that spans the world's recent period of repeated glaciations. The name pleistocene is derived from the Greek and ....

 in age, and the rock primarily consists of scleractinian

Scleractinia

Scleractinia, also called stony corals, are exclusively marine animals; they are very similar to sea anemones but generate a hard skeleton. They first appeared in the Middle Triassic and replaced tabulate and rugose corals that went extinct at the end of the Permian...

 coral, such as Elkhorn coral

Elkhorn coral

Elkhorn coral is considered to be one of the most important reef-building corals in the Caribbean. This species of coral is structurally complex with many large branches. The coral structure closely resembles that of elk antlers. These branches create habitats for many other reef species such as...

 and Brain coral

Brain coral

Brain coral is a common name given to corals in the family Faviidae so called due to their generally spheroid shape and grooved surface which resembles a brain...

.

The Hurricane Monument, commemorating victims of the Labor Day Hurricane of 1935

Labor Day Hurricane of 1935

The 1935 Labor Day Hurricane was the strongest tropical cyclone of the 1935 Atlantic hurricane season, and one of the most intense hurricanes to make landfall in the United States in recorded history...

, and located at mile marker 82 on US Route 1 near Islamorada

Islamorada, Florida

Islamorada, a "Village of Islands," is an incorporated village in Monroe County, Florida, United States. It is located on the islands of Tea Table Key, Lower Matecumbe Key, Upper Matecumbe Key, Windley Key and Plantation Key in the Florida Keys....

, is constructed of keystone, as is the David W. Dyer Federal Building and United States Courthouse

David W. Dyer Federal Building and United States Courthouse

The David W. Dyer Federal Building and U.S. Courthouse, formerly known simply as the U.S. Post Office and Courthouse, is an historic United States Post Office and federal courthouse of the United States District Court for the Southern District of Florida located at 300 Northeast 1st Avenue in...

.