The main building is the central ubosoth, which houses the statue of Emerald Buddha. The legendary history of this Buddha image is traced to India, five centuries after the Lord Buddha attained Nirvana, till it was finally enshrined in Bangkok at the Wat Phra Kaew temple in 1782 during Rama I's reign (1782–1809). This marked the beginning and raise of the Chakri Dynasty of the present Kingdom of Thailand (the present head of the dynasty is King Rama IX.)

The Emerald Buddha, a dark green statue, is in a standing form, about 66 centimetres (26 in) tall, carved from a single jade stone (Emerald in Thai means deep green colour and not the specific stone). It is carved in the meditating posture in the style of the Lanna school of the northern Thailand. Except for the Thai King, no other person is allowed to touch the statue. The King changes the cloak around the statue three times a year, corresponding to the summer, winter, and rainy seasons, an important ritual performed to usher good fortune to the country during each season.

While legend traces this statue to India, its rich historical records dates its finding in Cambodia in the 15th century, moved to Laos in the 16th century and then to Vientiane where it remained for 215 years, and finally to Thailand in the 18th century. Considering the long history and Nagasena's (a Brahmin who became a Buddhist sage and lived about 150 BC) prophesy that the Emerald Buddha would bring "prosperity and pre-eminence to each country in which it resides", the Emerald Buddha deified in the Wat Phra Kaew is deeply revered and venerated in Thailand as the protector of the country.

History

The Emerald Buddha statue originated in India as explained in the legend, but it is also linked to first vassal Kingdom of Cambodia. The image disappeared when Burmese raiders sacked Ayuttaya also spelt "Ayudaya" and the image was feared lost.

Continuing with the legend of the saint Nagasena of India, after remaining in Pataliputra (present day Patna) for three hundred years, the Emerald Buddha image was taken to Sri Lanka to save it from a civil war. In 457, King Anuruth of Burma sent a mission to Ceylon with a request for Buddhist scriptures and the Emerald Buddha, in order to promote Buddhism in his country. These requests were granted, but the ship lost its way in a storm during the return voyage and landed in Cambodia. When the Thais captured Angkor Wat in 1432 (following the ravage of the bubonic plague), the Emerald Buddha was taken to Ayutthaya, Kamphaeng Phet, Laos and finally Chiang Rai, where the ruler of the city hid it. Cambodian historians recorded capture of the Buddha statue in their famous Preah Ko Preah Keo legend.

The Emerald Buddha reappeared in a chance discovery in Chiang Rai in 1434, after a lightning storm struck a temple. The Buddha statue fell down and was chipped. The storm had washed away some of its mud plaster covering (mud coat or stucco used to be laid to safeguard valuable Buddha images). The monks, after removing the plaster around the statue, discovered that the image was a perfectly made Buddha image from a solid piece of Jade, a precious stone. After that, the image moved around a few temples in Northern Thailand. It was then moved to Chiang Rai, then Chiang Mai, from where it was removed by prince Chao Chaiyasetthathirat to Luang Prabang, when his father died and he ascended the throne of both Lanna and Lan Xang, in 1551. The statue remained here for twelve years. Chaiyasetthathirat then shifted it to his new capital of Lan Xang in Vientiane in the 1560s. He took the Emerald Buddha with him and thereafter the image remained in Vientiane for two hundred and fifteen years until 1778. In the early 18th century, the Kingdom of Lan Xang was divided into 3 different kingdoms; Vientiane, Luang Prabang, and Champassak.

King Taksin of Thonburi (Siam, now Thailand) was crowned king in 1768 (he had defeated the Burmese), reigned for fifteen years, united the kingdom and expanded its territorial jurisdiction. Chao Phya Chakri (Chakri is a title) a renowned army general and associate of Taksin, in 1778, defeated the Vientiane and shifted the Emerald Buddha from Vientiane to Thonburi where it remained till Taksin's death. It was then deified in a shrine close to Wat Arun. Chroniclers mention that Taksin had become senile and consequently he was put to death by Chao Phra Chakri. Chao Phra Chakri then took over the reins of the Rattanakosin Kingdom. He adopted the title Rama I and shifted his capital across the Menam Chao Phra river to its present location in Bangkok. The Emerald Buddha was also moved across the river with pomp and pageantry and deified in the temple of Wat Phra Keo.[1] It resides in the Wat Phra Kaew in the precincts of the Grand Palace. Rama I, after he moved the capital from Thonburi to Bangkok, got the temple consecrated in 1784. The King had ordered replacing an old temple that existed at this site in the 16th century, by building the new temple, as part of his new capital; both were built concurrently. It was built as an exclusive temple only to display holy buildings, statues, and pagodas. The formal name of Wat Phra Kaeo is Phra Sri Rattana Satsadaram, which means "the residence of the Holy Jewel Buddha." 

Phibunsongkhram, a World War II hero of Thailand, the Prime Minister, Commander-in-Chief of the Army and Supreme Commander of the Armed Forces in 1941 had signed a formal treaty of alliance between the two Buddhist countries of Thailand and Japan in the divine presence of the Emerald Buddha in the wat. He had royal ambitions of shifting the capital from Bangkok to his home town Phetchabun along with the Emerald Buddha. He later gave up his plan under public pressure and also fear of bombing during the war.

However, there are also claims that the statue was originally in Sri Lanka. Art historians of Thailand claim that it was cast in the 14th century in Thailand only. All these theories are discounted on the grounds that none of the historians could get a close look at the statue.