Hi minmin001,
Latex cores are made of rubber that has been foamed into the cores used in a mattress. It is made using two different processes (the Talalay process and the Dunlop process) and either of these can use either natural rubber (from the Hevea Brasiliensis tree) or synthetic rubber (Styrene Butadiene) or a blend of the two along with the other compounds that are needed to make the foamed core.
You can read more about the different types of latex in this article and in post #6 here.
For a mattress that contains 9" of blended Talalay latex, the Malaysia is great value IMO.
Phoenix