This week was a good time to be invested in cryptocurrencies. And, it follows, the period was also a fine one for holders of crypto mining (plus mining equipment) stocks.

According to data compiled by S&P Global Market Intelligence, sector leaders Marathon Digital Holdings (MARA 2.21%) and Riot Platforms (RIOT -1.49%) both landed well in positive territory. They rose a respective 14% and 8% over the week. Other winners included Canaan (CAN -1.74%), with a 13% pop; CleanSpark (CLSK -1.59%) and its 6% advance; and Cipher Mining (CIFR 1.58%), up a meaty 28%.

The October crypto surge

For much of October, cryptocurrencies and related assets have generally been on an upswing. The most immediate catalyst for this is, somewhat oddly, a type of crypto investment that isn't actually available on the market yet.

This is the spot cryptocurrency exchange-traded fund (ETF). Unlike other crypto ETFs currently on the market, which artificially track prices of investments like Bitcoin, these securities would see their managers invest directly into coins and tokens. Buying these ETFs would be far easier than traditional crypto investment, which involves the owning and management of specialty digital "wallets."

Many crypto bulls believe that we are on the brink of spot crypto ETFs coming to market; hence the heightened optimism and ballooning prices.

This isn't wild speculation, either -- this week kicked off with the news that a judge had issued a ruling in favor of Grayscale Investments' ambitions to turn its Grayscale Bitcoin Trust into an ETF. This doesn't guarantee the transformation will ultimately occur, but it is an important potential step on the way there.

Previous to that, a bonfire was lit under crypto and mining stocks when a false news item spread about BlackRock's proposed Bitcoin ETF. This claimed that the instrument had won regulatory approval; although the posting was quickly proven to be inaccurate, it added to the general feeling in the crypto community that such ETFs are getting closer to hitting the market.

Perhaps this is an unsustainable rise

This optimism has snowballed over recent weeks for a fairly long-tail rally in both cryptocurrencies and the assets that rely on them. In fact, early in the week Bitcoin hit its all-time high, briefly trading at over $35,000 apiece. 

So even though that bullishness is justified to some degree, these recent surges in prices might not necessarily be sustainable. Investors considering a crypto buy or several might be wise to take a breath to consider this and be very careful where they are allocating their funds.