The stupa is by far the earliest and architecturally the most significant Buddhist expression. Burial mounds were already in use at the time of the Buddha and he suggested that a stupa be built for his remains at the intersection of four major roads - i.e. in a public place. After his death the relics were divided into eight portions and perhaps the earliest datable stupas are those at Kusinara, site of his cremation, and the one raised by his family. Presumably these were built not long after his death. Originally stupas were little more than a mound of earth raised over the remains of saints, kings, etc. but over the centuries they have been gradually transformed into major works of art.

The basic elements of construction had evolved after only a few centuries of development and the main four are: the base - usually square; a 'hemispherical' dome; a reliquary - often on top of the dome including a spire (often a stylised umbrella) and, the jewel or crown. There is a great deal of symbolism and stylistic developments that have come to be associated with stupas.

With the spread of Buddhism, the need many people had for a tangible focus for worshiping the Buddha as a semi-divine and then a divine figure gradually developed. The worship of stupas increased parallel with this. Stupas were also made on a small scale as (portable) objects for devotional worship and/or as containers to hold sacred relics. These reliquary stupas (and stupas generally) might contain human remains and an assortment of beads, crystal, pearls, gem stones, and gold or silver in various forms are sometimes found in the relic chamber. The mixture of sacred and precious often seems haphazard, suggesting that the intention of the donor was of primary importance.

from www.buddhamind.info