The three most popular topping mediums for modern fire pits are lava rock, glass beads, and stones. Though they all share the qualities of being fireproof and somewhat homogeneous in size, there are a few key differences.
Lava rock and glass are both good dispersion mediums – they do a good job of breaking up and distributing the fire in a pleasing way. This is due to the fact that they can be smallish, uniform particles that create air channels for gas to seep around. Stones are often larger and need to be placed more strategically so as not to impede the flow of gas and create a less aesthetically pleasing fire. With stones, it is necessary to use one of the other mediums as a base layer and simply place the stones on the very top.
(A note about stones: Not just any stones can be placed in a fire. In fact, it can be dangerous to use stones that are not specified heat-resistant, as they can crack, pop and even explode when heated.)
Of lava rock, glass and stones, each absorbs and retains heat quite differently. Lava rock is very porous and hardly holds heat at all. Stones are at the other end of the spectrum. They can hold and radiate heat long after the fire is extinguished. Aesthetically, of course, the three mediums are very different. Lava rock has a charred, craggy look of coal, stones have a peaceful, organic quality and glass is slick, reflective and the most overtly decorative of the three.
Okay, so I know what you’re thinking: “Yes, but isn’t fire rising magically out of rocks or glass, no less ‘artificial’ than ceramic logs?” Well, although in the practical sense this may hold some truth, there are some fundamental differences, aesthetically and ideologically. If we consider the main principles of modern design: simplicity of form and material and functional honesty, then the differences are clear.
The goal of the modern fire pit is to be an outdoor fire feature that provides warmth and aesthetic beauty, utilizing available safe and clean-burning fuels (in a way that embraces modern design ideals). The fundamental purpose of a gas or propane fire pit with “traditional” styling is to emulate something else, the “real thing.” Modern fire pits use fire as a medium, a material that has aesthetic and practical qualities, and as such, the key to designing a beautiful fire pit is to find the functional and visual balance of the different mediums. With natural gas and propane fire pits, toppings are technically necessary to achieve an aesthetic result: they serve to enhance the fire as well as to visually complement it. They are an essential material counterpoint to a modern firepit.