Fire Glass and Lava Rock

What to Know Before You Buy

Five decisions shape the media in a gas fire pit.

• Glass or rock. Pick the media your burner manual allows, then choose on looks. Glass is reflective and goes on thin, and lava rock reads as natural stone. Fire glass or lava rock compares depth, cleaning and what each burner allows.
• Depth. The burner manual wins over the bag. HPC's and The Outdoor Plus's insert manuals say to cover the burner with 1/4 to 1/2 inch of glass, while The Outdoor Plus's 2024 quick reference guide does not recommend glass, sand or fire media in any propane fire or fire-and-water feature. On propane, ask the maker which document applies before you size glass. See the outdoor fire bowl guide.
• Quantity. Glass is sold by weight and lava rock by volume. The fire glass calculator turns a pan size into pounds or cubic feet, and the guide on burner kits, drop-in pans and inserts covers pan sizing.
• What shipped with the piece. Prism includes lava rock with its Tavola and Moderno models, 27 or 54 lb on most and six 27 lb bags with the Moderno 70, and Bull's Small Square pit includes lava rock, so you may not need to order any.
• Wind and winter. Firegear calls a wind guard optional, and whether gas fire pits give off heat covers what makers say about glass guards. For drains, covers and wet media, see how to winterize a gas fire pit.

Sizes, Output and Fuel

Lava Rock Direct lists 11 products in five size ranges, from 1/8 to 9 inches. Small rock is 1/8 to 1/2 inch and medium is 1/2 to 1 inch, both in black, red and bronze. The tumbled products come in gray. HPC lists ten glass colors, from Clear to Cobalt Blue and Copper.

The table assumes a 2 inch fill, a common depth and not a rule, so your burner maker's chart overrides it. Glass works out to about 90 to 96 pounds per cubic foot of pan filled, from the published coverage of HPC and MSI, a glass supplier. No maker publishes a weight per cubic foot for lava rock, so the fire glass calculator reports cubic feet only.

Depth over the burner is a separate limit, and it depends on the fuel. HPC's and The Outdoor Plus's insert manuals cap glass at 1/2 inch on propane because more can create back pressure that pushes gas out of the air mixer, where it pools under the pit, and both say a propane unit must be tested with the media in place. The Outdoor Plus's quick reference guide goes further and does not recommend glass in any propane feature, so its two documents disagree. Firegear caps top media at 1 inch over its Pro burners.

Who Makes Them

These makers' own documents show they supply fire media. The order is not a ranking.

Lava Rock Direct, of El Cajon, California, is a wholesale supplier of lava rock. It says the rock is mined, cleaned and packaged in the USA, without naming a state or quarry, and it supplies businesses wholesale, with custom and private-label bagging. It lists its rock for gas fire pits, fireplace inserts and fire tables, and it does not list fire glass. Its claims that the rock withstands extreme temperatures without cracking are the company's own.

HPC Fire Inspired, of Miamisburg, Ohio, lists fire pit glass in a media category beside its inserts, burner kits and ignition systems.

Rules and Placement

Each burner maker lists the media it allows. The Outdoor Plus names fire-rated lava rock or other igneous rock, fire glass approved for fire features and man-made stone for fire features. HPC and The Outdoor Plus bar river rock, gravel and pebbles, because trapped moisture boils and fractures them. They also bar a mesh or screen under the media, call concrete logs not recommended, and say media goes in the pan only, never in the enclosure cavity.

Wet media pops, and the makers agree on the cause. Firegear calls it thermal spalling: moisture trapped inside wet media cracks it or pops it out under rapid heat, glass included. If lava rock is wet, HPC and The Outdoor Plus say to burn the pit 45 minutes before coming within 15 feet, Firegear says to light it and watch from a distance for at least 30 minutes, and Prism says not to light the feature while the media is wet. Firegear says a cover can lessen spalling, but heavy rain and humidity may still cause popping.

HPC and The Outdoor Plus want the pan and burner at least 2 inches below the top of the enclosure to keep the flame from blowing out, and they list a wind guard as a fix for a flame that keeps going out. After changing media on a propane unit, have a qualified installer test for back pressure with the media in place. Run your pan through the fire glass calculator, check your local code in the fire code library, and follow the installation manual for the exact unit.