Mm'x crystals were objects of both artistic and practical use created by the Aveletians. They appeared as brightly-coloured crystals with no fixed shape. At the same time, they did not appear to change shape either, although their colour did appear to vary, also emitting a soft glow. This was because only part of the crystals were visible to three-dimensional eyes.
When the Aveletians were tricked into boarding the Sou(ou)shi ship in expectation of their relocation to a safer planet than their own dying one, they left a sculpture of mm'x crystals as a gift for their hosts. When the First Doctor observed the work, he thought that it suggested an ending or a completion.
The Doctor then gathered more than 3000 of these crystals from the ship and used them to escape, with the Sou(ou)shi unaware of their powers. After stealing a Gowenn ship from the shuttle bay, the Doctor used the crystals to buffer his re-entry into the Venusian atmosphere by dimensionally displace the thermal energy of the atmospheric friction, allowing the safe landing on the planet's surface. Landing on Sisikhigu’s Beach, the Doctor did a quick calculation if a quotient for energy absorption and multidimensional displacement to work out that he had not travelled far in either space or time as a result of his use of the crystals. (PROSE: Venusian Lullaby)