"Hard times" means "times of change". Times of war. Times, when we focus our eyesight into the current problems. There aren't that many other topics we pay attention to aside of solving the problems when they arrive. Can buying a jewelry be a solution?

Surprisingly, yes, it can. The jewels have always been a symbol of prestige, wealth, of authority and power. For most of the people today, a jewel is a mere decoration. Even so, they haven't lost their importance. Would you go to a wedding, a cocktail or an anniversary without the proper jewelry? Would you not gift a jewel? 
In either cases we are searching for a rare and beautiful piece, and if possible - for a good price.

Can You Invest In Jewelry?

Expensive jewelry should be bought only for long-term purposes.  The purchase of an expensive art jewelry would result in doubling or trippling its value over the years. There are two reasons for that. First, the longer the artist works in their field, the more famous and well sold are they. Their works have more value later than at the beginning. Second, the price of the material used to create the piece also rises with the time. For example, back in 1998 a ounce of gold was sold for under $292. Twenty years later, in 2018, the price has risen to $1281.65 (Source: https://www.cotizacionrealoro.com/historicos-graficas-evolucion-precio-oro).

Thus seen, buying jewelry should be regarded as a long-term investment, related to the prestige, inheritance, or for collecting purposes and not for resale.

An interesting fact: Russia and other post-Soviet countries have the tradition of gifting gold and gems jewelry to little girls. These pieces form a kind of tangible tresor for their owners.

The sanctions over trade and banking placed by the EU to Russia have practically left with no means of existence a great number of Russian citizens living on the European continent. For many of them, being in possession of a life-long accumulated jewelry might mean a survival today.

How About Short-term Purchases?

I personally wouldn't buy to resell. The most of the jewelry production loses at least 50% of its  selling price at the moment the buyer leaves the shop. What is being sold in the most jewelry stores is a kind of jewelry confection with no great value.
Reselling this will, at most, recover only a part of the price of the material the piece is made of.

Buying High Jewelry in Hard Times

I would opt for that. The currency value falls constantly in times of crisis and inflation, while art and decorative objects gain value. Also, don't forget that we are living in times when the banks are looking for a way to convert the physical money into digital. Jewelry can be a response that allows you to solve a possible solvency problems. Thus you will not only have pieces to adorn your looks, but also a long-term tranquility. This kind of financial stability doesn't really have an alternative in other active possessions.