Should you believe in AIM Cinderella stories Sirius Minerals plc and Boohoo.Com plc?

Fairy tales generally come with a more exciting plot than building a fertiliser mine under the moors in Yorkshire, but despite lacking dragons or talking snowmen, shareholders of Sirius Minerals (LSE: SXX) have undoubtedly enjoyed the company’s year-to-date performance more than any Disney movie they’ve seen.

Share prices have rocketed 150% since January after the company jumped a series of hurdles to receive all necessary planning permissions and then topped off the good news by lowering projected capital expenditures 18% from $3.5bn to $2.9bn

That said, there are still significant challenges that remain before Prince Charming fits Sirius with the glass slipper. First among these is raising the cash necessary to begin work. The first stage alone is expected to have a price tag of $1.09bn and will be raised through roughly equal amounts of debt and equity, which could mean significant dilution for current shareholders.

Raising the necessary funds may not be going as well as hoped, either. Management had maintained throughout the summer that construction was targeted to begin this month but we haven’t had any major updates on how the critical stage one fundraising is progressing.

Even if the company makes a surprise announcement in the coming days, best-case scenarios don’t see first production until 2021. That’s not a problem for long-term investors, but it certainly leaves a lot of time for problems to arise.

The question that looms largest in my mind is what the market will be for Sirius’ product, polyhalite. There is currently no liquid market for the fertiliser, so it’s difficult to judge what demand will be like tomorrow, much less five or 10 years from now. The company believes it will be largely insulated from the current downturn in potash prices, but without disclosing terms for off-take agreements we simply have to take it at its word.

Sirius has done well to hit self-imposed targets thus far, but the trickiest bits lie ahead for the firm and with major capital raising to come and questions as to the marketability of its product, a happy ending is anything but certain for this AIM darling.

Will the spell be broken?

The investing fairy godmother evidently visited Boohoo.Com (LSE: BOO) and turned what was a mere pumpkin into a gilded carriage as share prices are up over 180% in the past year. This share price growth hasn’t simply come from nowhere, of course, as the company is forecasting a roughly 30% rise in year-on-year sales for the current financial year.

Still, investors may have gotten ahead of themselves in anticipating the next Asos as shares now trade at an astronomical 59 times forward earnings and the firm has a market capitalisation of over £1bn, despite booking only £15.6m in pre-tax profits last year.

Bullish investors would be right to point out that growth hasn’t been a problem for Boohoo so far, so maybe this lofty valuation isn’t too far fetched. However, I remain largely unconvinced that it will be smooth sailing for Boohoo in the long term.

Selling cheap clothing to 16-24-year-olds isn’t only a relatively low margin business but also one that requires constant innovation in design and marketing. Eighteen-year-old shoppers today may love their Boohoo-branded jeans but history has shown their tastes are fickle and there’s no guarantee they won’t move on to the next fad in a few years time.

One high growth company that has proven it’s more than a fad is the Motley Fool’s Top Growth Share. This firm has increased sales every year since going public in 1997 and doesn’t look anywhere close to slowing down.

Even with share prices up over 250% in the past five years the Fool’s top analysts see the potential for shares to triple again in the coming decade.

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Ian Pierce has no position in any shares mentioned. The Motley Fool UK has recommended We Fools don't all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors.