Moissanite

Moissanite is the name for transparent silicon carbide (SiC) of gemstone quality.
It may be an especially interesting (if not the most interesting) base material for gemstone based metamaterials in gem-gum products because of its set of peculiar properties.

Resistance against heat

Compared to diamond and its polymorphs such as lonsdaleite moissanite has a much better resistance against high temperatures.
Diamond is only metastable at room temperature and converts to the lower energy state of graphite if it's heated up far enough. That is not the case with moissanite due to silicon not wanting to form graphite like sheets.

Resistance against larger scale fire

Compared to diamond moissanite is more resistant against oxidation and fire. Because it contains the nonvolatile (slack forming) element silicon. Moissanite is not a fully oxidized gemstone material (like e.g. quartz or leukosapphire is) thus it is not immune to oxidation an indeed gets its surface nanosctructure destroyed when contacting an oxygen containing atmosphere at somewhat elevated temperatures. But …

• simple sealing against entry of atmosphere can prevent that
• there is no oxidation on a larger scale because a protective slack layer out of quartz-glass is formed that prevents further oxidation and runaway fires.

While diamond cracks splinters and burns under a strong flame moissanite just turns yellow and back to clear again when it cools again.

Neo polymorphic structure control

Natural moissanite and thermodynamically synthetic moissanite come with a rather random layer order (not ABAB hexagonal or ABCABC cubic but something in-between). When it's produced via mechanosynthesis instead this layer ordering can by precisely controlled. See neo-polymorph.

Interplanetary applications (Venus)

On Venus the silicon for the silicon carbide base material would need to be retrieved from the hot surface. This shouldn't be too difficult though because to get the element silicon one basically can haul up almost any random rock that lies around loosely. The rocks can then be chemically processed at 50km height where machines and especially humans can easier operate. No serious ground mining (with drills or so) required.