Quartz is silica. Glass is made of silica, but as you said can have some other ingredients mixed in. I found an Encylclopedia Americana article that lists the constituents of "dense flint glass" as 38% silica, 8% potash and 54% lead oxide (PbO). Another site lists the density of lead oxide as 9530 kg/m^3, and the density of potash as 1281 kg/m^3. So at those percentages, "dense flint glass" would have a density of approximately 7256 kg/m^3. Still less than steel, but pretty close. Wiki claims the percentage of lead oxide can be as high as 60%, so that might make it more dense than steel.