[Kaijo]

This is true with some qualifications. If you're interested in the kind of investment grade diamonds that a major auction house would deal with, then you're looking at heavy weights and/or fancy colors that synthetics can't reach yet. In the diamond trade the word "paragon" is sometimes reserved for flawless or near-flawless stones above 100 carats, of which there is a long list of famous examples, but the largest gem grade synthetic is still around 30 carats I believe. Vivid colors top out at much lighter than that. I guess we'll be able to outdo nature within a few decades though (as far as terrestrial diamonds go, anyway -- I seem to recall reading somewhere about the discovery of moon-sized space diamonds).

[A_D_E_P_T]

Vivid colors are a trivial engineering problem, and one the Chinese have already cracked. Screenshot: https://ibb.co/s6gWTy1

Prices are dropping like a rock from a high tower, and colors and other options are becoming more available. Within 10 years you'll be able to buy virtually any diamond you like, in any common color, for less than a 2ct stone would have cost in 2014.

Also, if you really like huge gems, you can buy a moissanite today at 1000ct. Even on Amazon.com there are examples at 100ct. https://www.amazon.com/Gemonite-15CT-100CT-Moissanite-Colorl...

This would have been unthinkable 10 years ago. Things in the diamond and gemstone business are changing fast.

[bluGill]

Though if you are interested in investment grade diamods I'd say it is time to get out - diamonds have never been as rare as investors like to pretend, and things are going to get worse.

[canucker2016]

Diamonds have been used as plot devices in movies and TV shows as a liquid asset for criminals.

*Spoiler Alerts*

Marathon Man (don't watch if you're squeamish about dental practices) with Dustin Hoffman and Laurence Olivier. see https://en.wikipedia.org/wiki/Marathon_Man_(film)

Also the 2021 Irish/Belgium TV series, Hidden Assets, involves diamonds.

[djtango]

A diamond the size of a moon? Does that mean it's a single molecule of pure carbon the size of the moon? I wonder what effects gravity has at that scale

[indoordin0saur]

I don't know about moon sized but there are solar systems out there where carbon is more common than silicon. In such a system if you had a terrestrial planet then you're likely get diamonds instead of quartz being the most common mineral in the crust. You also might possibly get diamonds in an octagonal crystalline form which are theorized to be far stronger than the diamonds we have here on Earth.

[elihu]

The density of diamond is about 3.5 g/cm^3. The Earth's moon has a density of around 3.3 g/cm^3, so if you replaced it with a diamond of about the same size, it actually wouldn't be all that different in terms of gravity. Solar eclipses would be pretty wild though.

[djtango]

Was more thinking about whether there are any interesting gravitational effects on the internal chemistry of a giant crystal. Say it is one single crystal, how would its centre differ from the surface?

[defrost]

Earth, Moon, or Big Fine Diamond, the gravity at the surface is 1 unit (for whatever normalised radius, density to get a 1 works) while the gravity at the centre is 0.

[A_D_E_P_T]

It's certainly polycrystalline rather than a giant single-crystal. It would contain lots of every other element that's soluble in it, to its limit of solubility, and whatever's insoluble or over that limit would have to form different mineral inclusions at grain boundaries.

[FactKnower69]

"Investment grade", hahaha! "Speculation grade" would be a much less pathetic way to articulate this absurd concept