Does fused quartz float?

No. At 2,200 kg/m³ it is denser than water.

Fused quartz

Density of fused quartz

2,200 kg/m³

2.20 g/cm³ · 137.3 lb/ft³

Pure silicon dioxide with nothing added, which makes it the extreme case: a thermal expansion coefficient of 0.55 µm/(m·K), about a sixteenth of ordinary glass, so a red-hot tube can be quenched in water without cracking. It also passes deep ultraviolet, which ordinary glass blocks, which is why UV lamps and photolithography optics are fused quartz. It has no melting point and only softens above 1600 °C, so working it needs an oxy-hydrogen flame rather than a normal glassblowing torch.

Properties

Density: 2,200 kg/m³ — 2.20 g/cm³
Tensile strength: 48–110 MPa
Young's modulus: 72 GPa
Thermal conductivity: 1.38 W/(m·K)
Hardness: 6 Mohs
Thermal expansion: 0.55 µm/(m·K)
Floats in water: No
Magnetic: No

What it is used for

Semiconductor furnace tubes and wafer carriers
UV and infrared optics
Halogen and arc lamp envelopes
Optical fibre preforms

How it compares

Water is 1,000 kg/m³. Anything with a shorter bar than water floats in it.

air 1.2 kg/m³

expanded polystyrene foam 20 kg/m³

rigid polyurethane foam 40 kg/m³

Water — the float line 1,000 kg/m³

fused quartz 2,200 kg/m³

tungsten 19,250 kg/m³

gold 19,300 kg/m³

platinum 21,450 kg/m³

Water is 1,000 kg/m³. Anything below that floats.
Material	Density	Relative to water
rigid polyurethane foam	40 kg/m³	0.04×
expanded polystyrene foam	20 kg/m³	0.02×
air	1.2 kg/m³	0.00×
fused quartz ←	2,200 kg/m³	2.20×
platinum	21,450 kg/m³	21.45×
gold	19,300 kg/m³	19.30×
tungsten	19,250 kg/m³	19.25×

Does it rust?

Fused quartz contains no iron, so it cannot rust. Whether it degrades depends on the material, not on oxidation.

Is it magnetic?

Fused quartz is not a metal and has no ferromagnetic response.

Common questions

What is the density of fused quartz?: 2,200 kg/m³, or 2.20 g/cm³.
Does fused quartz float?: No. At 2,200 kg/m³ it is denser than water.
Is fused quartz magnetic?: Fused quartz is not a metal and has no ferromagnetic response.

Sources

Typical range compiled from manufacturer specifications — At One Place

How these figures are compiled and checked