Hi Uday Kumar,
Silver comes under Non-Ferrous metal. For Non-Ferrous metal procurement SAP has a solution in DIMP. Please checkout the following slide.
http://help.sap.com/bp_fm603/BBLibrary/Documentation/370_Scen_Overview_EN_FR.ppt
Please note that precious metals are procured from the supplier on premium basis (Premium + LME Price [London Metal Exchange]). And pricing with the supplier is on agreement basis, not on daily basis. So while creating the PO, you have to enter the agreed premium and the LME price in the conditions.
Regards,
Asik.