Sandstone is a Vanilla block which can be mined from Desert and Beach Biomes. The block itself serves no purpose in any major Tekkit-based crafting recipes and is predominately a decorative material. The method of crafting sandstone has been slightly expanded upon with Industrial Craft 2, and can now be created by placing sand in a Compressor. Sandstone can also be converted back to sand using a Macerator.
A link to the Minecraft Wiki's page about Sandstone can be found here.