Fused Quartz is a customisable glass block, that also looks a bit nicer than the default vanilla glass block. Fused Quartz blocks also connect textures when there are multiple next to each other. Fused Quartz blocks are customized using Fused Quartz Frames.
Fused Quartz is created by putting 4 Nether Quartz inside an Alloy Smelter, and requires 5000 RF to be produced.
see: Crafting Guide
To use fused quartz blocks in the default state, you simply place them in the world. Multiple Fused Quartz blocks placed next to each other will connect, producing a seamless window. To change the frame (border) of the fused quartz, you have to right click the Fused Quartz block with a painted Fused Quartz Frame, painted using the Painting Machine.