Personally after reading the article, I came to the conclusion as they are not referencing a crystal as in Quartz Crystal, but as in Crystalline Form under unknown pressure they think may be there.

Early statements in the study use alot of speculation as a fact when worded like the opening sentence "There's a giant crystal buried deep within the Earth, at the very center, more than 3,000 miles down" and then we get wording like "even though the temperature is believed to exceed 11,000 degrees Fahrenheit, hotter than the surface of the sun"....

The article further falls apart with statements of supercomputers can't quite do these computations yet..... but were guessing anyways.

To this closing befuddlement: Could an iron ball 1,500 miles across be a single crystal? Unheard of until this work, the idea has prompted realization that the temperature-pressure extremes of the inner core offer ideal conditions for crystal growth. Several high-pressure laboratories have experiments planned to test these results. A strongly oriented inner core could also explain anomalies of Earth's magnetic field, such as tilted field lines near the equator. "To do these esoteric quantum calculations," says Stixrude, "solutions which you can get only with a supercomputer, and get results you can compare directly with messy observations of nature and help explain them...

The Inner Earth is still complete speculation, and a computer will spit out ANY answer that agrees with the parameter set that it was programmed to analyze....BUT it sure won't tell you if you are missing data fields or bending the laws of physics at a minimum to get your funding money.

We still don't know about Temperature/Pressure Extremes and speculate as to if they exist within our core.